You don't rename it anything, what you have to do is replace the file that is in the folder with the new one. Also the new file has to have the same name as the old one.
Like, to replace the Trunks one, you'd take out the old file, which is called trunks.mdl (i think) and put in the new file, and then make sure the new file is called trunks.mdl.
I'm gonna start editing my post, incase a mod considers this spam.
Like i said before, back up the original model, put it in a folder in My Documents or something, incase you ever want it back. That's what i did.