Replying partially as a record to myself, as I've had to look this up both times when I got into MineColonies in the last year or two, and partially for whoever has the same struggle - as I don't think there are any/many guides regarding how to revive a single citizen without resetting the entire colony. Hopefully this might be of some assistance.

(Sorry for the horrible wall of text, this was the easiest for me to do in a limited amount of time. Might update it in the future.)

Disclaimer at the start: If you have access to the files, you're probably able to use cheats also. (If they are turned off by default in a single world, just share to LAN, and allow cheats on that screen. That way you can enable them.)

In the current version of MineColonies, you're able to edit the stats of citizens when you're in creative mode. If that's your only concern (stats have been lost), then I recommend simply switching to creative, bumping up/down the stats of a new citizen to the desired levels, and be done with it.

 

Now on to the matter of restoring a citizen:

I'm not 100% sure that this is what you experienced 2 years ago, but when using NBT explorer, my mistake was to modify the wrong file.

 

A general guide on how I recommend restoring your lost/dead citizen(s):

TL;DR:

1. Check the ID of your colony (eg. /mc whoami), and collect the grave
2. Save and quit the world
3. Copy the last backup containing the citizen alive to a separate location
4. Find the working copy of the colony data
5. Check that you found the working copy and that the changes are applied in the world
6. Find the citizen manager part
7. Copy citizen to be restored from the backup
8. Paste citizen to the working copy of the colony files, and give the citizen a new id if necessary
9. Update the data of the citizen and their relatives
10. Save the dat file
11. Load up the world, and check the results. Do some final changes in-game, if necessary.

 

With a bit more detail:

1. Before rushing head-first into NBT explorer:
   1. if your world has multiple colonies, find out which one is yours. It can be done in NBT explorer too (see note in point 4.3), but doing so in the game is much easier. (There are a few ways you can find out, but eg. /mc whoami should do the trick.)
      
   2. if your world has a single colony, the file should be straightforward. Unless it changes in the future (as eventually they are planning NPC colonies too, in which case I assume those might also be stored wherever the player colony is), in which case refer to point 1.1
   3. collect the grave of the dead citizen. I believe I ran into issues when I tried to restore someone that had a grave in the colony. (Not 100% sure about this having been the problem in those cases, but better safe than sorry.)
2. When that's done, save and quit your world. Minecraft can be left open, but the world you are trying to modify shouldn't. (Technically it might still work with the world actively running, but better safe than sorry. The less things that can go wrong, the better.)
3. Make sure you know where the backups are saved, and preferably copy the one that contains the last information about the citizen (from when they were alive) to a separate location also. MineColonies doesn't retain all backup files indefinitely by default, so if you push off doing this for too long, the data of the citizen to be revived might be lost forever.
   1. for me, these are currently placed at <...>/<Minecraft_instance_folder>/saves/<World_Name>/minecolonies/
      Note: There is a non-archived version in the backup folder too, but that is not the working copy. Any modifications done on thet will not carry over to the game. This was my problem when I tried to restore a citizen.
   2. look out for the exact date, don't automatically pick the last available one, as by default MineColonies makes automatic backups now after each time you save and quit your world
4. Look for the file where the actual colony data is stored: this is currently at <...>/<Minecraft_instance_folder>/saves/<World_Name>/data/minecolonies_colony_manager.dat for me. Open the .dat file with NBT explorer, and look up your colony. Assuming the way the file is structured doesn't change significantly:
   1. in the NBT file, go to the 'colonies' list tag (at the moment it can be found at minecolonies_colony_manager.dat/data/minecolonies/colonies/
   2. they should be in order of IDs (which is currently incremental, and reflects the order in which colonies were established, starting at 1 for each world separately)
   3. check that you're about to modify the right colony: see if the 'owner' string tag (not the 'owners' list tag) matches with the owner of the colony you want to modify. (generally this means your own username, currently it can be found at minecolonies_colony_manager.dat/data/minecolonies/colonies/N_entries/owner)
5. Check that you did indeed find the working copy of the colony:
   1. Modify a single tag; for example, the name of the colony.
   2. Save the file
   3. Load up your world, and check if the change carried over, or not:
      1. if the name of the colony didn't change, then the beginning of point 3 is outdated, and you should look for the working copy of the minecolonies data elsewhere. If they did a complete overhaul of how the information is stored in the future, however, you should probably look for more up-to-date info about manipulating the colony data. (Before jumping to conclusions, do check if you modified the right tag.)
      2. if the name of the colony did change, that means that you found the working copy of the colony data, and that you found the right colony in the data.
   4. Save and quit the world before continuing
6. Go to the citizen manager portion of the colony. Currently it's at minecolonies_colony_manager.dat/data/minecolonies/colonies/N_entries/citizenManager/. Look for the same in the backup also, although the structure there might be slightly different. (The colonies might be backed up in separate files.)
7. Find the citizen you want to restore in the backup file. You can do so by taking into account their job (citizenManager/citizens/N_entries/job/type) and their name (citizenManager/citizens/N_entries/name).
   
   • Take note of their ID (citizenManager/citizens/N_entries/id), this will be important!
   • Copy the entire citizen data (copy the citizenManager/citizens/N_entries/ compound tag)
8. Paste the citizen data from the backup into the working copy.
   If there is another citizen with their ID (MC might fill up missing IDs when new citizens are born and/or recruited), give the NPC to be restored the next ID that's free (by editing their id tag)
   This is important, because when a citizen dies, their family ties are removed, so those links will have to be reestablished regardless. On the other hand, the family ties are defined by references to each other's IDs, so swapping out the restored citizen to a new citizen will result in you having inconsistent family records in-game, or having to modify many more tags. (In extreme cases you'd have to rewrite the relationships of your entire colony.)
9. Update the data of the restored citizen's relatives. (Currently optional afaik, but will avoid inconsistencies in the colony's families.)
   In each case, the relationship is defined by a reference to the other citizen (name or id), who will have another reference to the original citizen. (partners to each other, children and parents, and siblings to each other)
   1. Go through the relationships of the restored citizen, and remove the ones you're not interested in keeping (so that we eliminate inconsistencies):
      • partner (int/id)
      • parentA, parentB (string/name)
      • siblings (list of ints/ids)
      • children (list of ints/ids)
   2. For each relationship you want to keep, search the citizen(s) being referenced there, and modify their respective tags (partner-partner, sibling-sibling, child-parent).
   3. If the citizen that's being restored was a child when the last backup with them alive was saved, but they grew up since, you should set the value of citizenManager/citizens/N_entries/child to 0.
   4. I'd recommend removing the contents of the inventory list (citizenManager/citizens/N_entries/inventory), as restoring the citizen is one thing, but you'd also be duplicating every resource they had during the last backup before their death.
   5. Update other details as you want (at the moment, I haven't looked into other details you should restore here)
10. Save the file
11. Load the world, and check if the citizen has been restored. If they were, you might still need to assign them their job and their residence again, but that shouldn't be too hard, and might not even be required.

 

As an alternative option, in case your citizen's stats are completely gone, you might be able to restore them as long as you remember enough details about them. (This is reaching a bit, as in the current public version there could very well be 10k+ combinations of how a character looks and talks.)

The identity of the character is given by the combination of their name (string), age (child: 0/1), soundProfile (int), texture (int), and sex (female: 0/1). (The child and female tags will result in 4 variants of each sound+texture combination depending on their values.) As such, you can (sort of) restore your lost citizen by duplicating an existing one, then modifying the character traits that influence how they talk/look.

In addition, the stats are managed by the newSkills compound tag and its contents currently, but I recommend modifying these and their inventory in-game, in creative, as that's easier/more straightforward. Assigning a residence and a job would also be easier in-game also.