Changing mods

Was wondering if it is possible to tweak a mod for local use only.
I want to change sonething(if possible) in a mod. Because the mod is someone elses work i dont want to upload it and make it available for public use.
I simply want to change a small funtionality (if possible) for use in my solo game.

You can’t change someone elses mod, unless they give you the source for it.
Once it’s cooked, you can not reverse it.

I guess i am stuck with it as it is then