all i said was open the assets0.pk3 and take out the player.npc which you'd then need to put into a seperate .pk3 and put into your base folder. I may not have added that part but I though most people know that by now.
So here we go.
using win zip extract the player.npc out of the assets0.pk3
it'll extract and look like this 'ext_data/npcs/player.npc'
using note pad open the player.npc and change where it has playermodel player to playermodel (name of model you want) so like playermodel yoda.
save the file
click on the ext_data folder and using winzip zip it up.
you now have a zip file, rename that to a .pk3 (player.pk3)
drop that back into your base folder.
you haven't changed anything in the assets0.pk3 and if you want to get rid of the changes you made just delete the new .pk3 you made. Simple.
Sorry, my fault, i read it wrong and thought you meant to edit the actual assets0.pk3.
Easy mistake to make