lol!!! hehe.. why didnt you said so
... Download a model.. then Go to half-life\models\player . then select a character you want. and install The model you downloaded in once of those directory and OVERWRITE the old one!
*if you downloaded a model called ssjvegeta.mdl you need to copy and paste this to half-life\models\player\ssjvegeta and overwrite the old file..
*make sure you make a BACKUP of the old one!