"ESF/Models/Players" you can find all the model files there. Simply download a model file, for example called "ssj4goku.mdl". And you want to change the Super Saiyan Goku with it, then go to the folder "ssjgoku" in "ESF/Models/Players". Remove ssjgoku.mdl, and add ssj4goku.mdl in it. Then rename ssj4goku.mdl to ssjgoku.mdl.
Remember that the model name should always be as the original one. Other wise ESF will give an error about that it can't find the model file. Sorry for the sloppy reply though, i'm rather tired at the moment.