SSJAgentSaba said:
you replace them in your "models" folder, in your, steamaps/email/half-life/esf/models/ssj goku, whatever
To be more specific, if you want the SSJ5 model, you download it and its SSJ5.mdl, you have to change that.
Lets say you want the SSJ5.mdl to replace the ssj goku. You should have to goto steam\steamapps\youremail\half-life\esf\models\players\ssjgoku
In the SSJ goku part there should be a file called ssjgoku.mdl
Put the SSJ5.mdl in that folder, and rename the SSJ5.mdl to ssjgoku.mdl, and he will be ssj5 ingame