Well, thats quite easy:
1), Download a custom model, (example named ssj4goku.mdl)
2), go to "steam/steamapps/yourusername/game/ESF/Models/players"
3), Search any folder you want to replace the model with, so lets say ssjgoku, search the folder name SSJGOKU.
4), in that folder you'll see "ssjgoku.mdl", remove that one (or make a backup) and put ssj4goku.mdl in that folder. Once you have done that, RENAME ssj4goku.mdl to ssjgoku.mdl and you're done, play ESF and you see the new replaced model..