How to get the game files to modify the characters?

Sorry if this is obvious or something, I'm new to creating mods and don't know where to start and want to use Arkham City as a testing ground. I've searched this countless times and have only gotten the answer on how to change the skins in the actual game, not what I'm wanting to do lol

I'm wanting to get the characters and add or change to their models. Only problem is, I have no idea how to do any of that and can't find anything helpful.