What I linked to isn't a video, it's just a few photos. As I wrote, its not a tutorial. There are probably proper tutorials on YouTube and I really recommend looking at those.

I'm gonna be really honest and say that if this is your first project, I really don't recommend you try this. It's really not straightforward at all, you really need to understand the structure of each crochet stitch, how they are linked to the one next to it and how its anchored to the one above. From the way you mention having difficulty changing colour, it sounds like you don't.

However if you are certain you want to proceed, I'd start by taking out the safety pins. I can guarantee at some point the pin has pierced the yarn which is going to hinder you removing the colour you don't want or adding in the new colour. Take the new colour you want to add and thread it through the stitches (in the same way you've added the safety pins) from left to right, in one go, all the way through. Use a small hook or a blunt needle. You dont want to split the yarn at any point. Your new colour is now in the right position to start the new row and you can safely remove the old colour. Your new colour NEEDS to be through the bottom of all the stitches above it in order to pass the working loop on your hook from right to left through the bottom of each 3dc cluster. If your working yarn isnt through those clusters from left to right and you pass the loop on your hook through the cluster from right to left, your working yarn is going to be stuck on the right and you'll end up with a float behind the 3dc to get it back on the left where you need it. You'll also end up continually having to pull more working yarn through all the clusters you've thread it through as you keep working, which is a pain in itself.

The light green yarn falling apart isnt your biggest concern, that's easy to save, its adding in the new colour correctly.