Is it against the rules to edit the art you've purchased?
If not, who do you put as the artist, if you're selling it? Colab/them/you?

(I highly doubt it's illegal)

As far as I know, no, there's nothing illegal in editing art that you've purchased.
If you're planning on selling it, give credit to the artist that made it, with you being its editor.

There's nothing wrong with it if you own it already, it's your property. You can do what ever you want with it.

I wouldn't use collab as it implies original creation was 50/50 or a joint effort. I would simply credit the original artist and then mention you yourself made modifications.

If you decided to sell it up again, what I'm doing is write the original artist and add my name right after the original one (ex : Artist : Chris, reedited by Azion)