Its quite simple RDT-Zero, in the folder "half-life\esf\models\player" are all the Model folders located. Now, if you want to install a different model. Simple put the Ussj5.mdl in any folder, and go in it. Delete the original model you see, and rename Ussj5.mdl to the model name you have removed, so for example:
You want to change Ussj5.mdl with Goku in normal form (Goku with black hair). You would go to "half-life\esf\models\player\goku". You see "goku.mdl", remove that. And rename Ussj5 to goku. And you're done. Just remember the model name has to be exactly the same as it was in the folder. Otherwise it won't work.