alright, go to your ESF folder then to the Models folder then to player. pick which player the model replaces(like if you have a new piccolo model, go to the piccolo folder, or if you have a new super saiyan trunks then go to the ssjtrunks folder.
Rename the old file to have a .bak at the end(like piccolo.mdl to piccolo.bak. This is in case you don't like the new model.
Copy your new model to that folder and rename it to what the old model was called if it isn't called that already. like if you had a newpiccolo.mdl, you would simply rename it to piccolo.mdl
And that is how you add a new model.