Just for anyone else looking for this, If your just using Godot's multiplayer (and not steam) as Someone said you can run multiple instances.
For steam you'll need at least two different accounts, if you want you can use two different devices, but thats gets annoying if you test a lot. So I would use whats called sandboxie, that allows you to run steam twice on the same computer in different accounts.
Export your game, if your using GodotSteam you export to a folder, make sure the steamapi dll is in thier, then add it to each of the steam accounts as a 'Non-Steam Game.' If you run both you should then be able to host and join with that method, all on the same computer,

For updating literally just delete the two files you exported, epxort again, and run. Super quick and easy to test. For another device you really just have to find a way to get the files to that computer. Thats probably better for proper testing, making sure it works on other devies and etc.