NEVER ALTER YOU ASSETS FILES!
If you do you won't be able to join "pure" servers or you might even corrupt the file(s) so your game won't work.
If you want to change something in the assets files you override your assets files by making a "mod" (.pk3 file). Put what you want to change in the mod's .pk3 file with a name that starts with a letter B through Z and the game will load it after the assets files (it loads .pk3 files alphabetically), overriding what it loaded from the assets?.pk3 files.
The .pk3 file must store the new files with the same directory structure as the assets files. So if the file you want to change is jawa_head.jpg, and in the assets?.pk3 file it is in the \models\players\jawa directory, the mod .pk3 file has to have the new jawa_head.jpg file in the \models\players\jawa directory inside the mod .pk3 file.
The easiest way to do this is to search
www.jk2files.com) ,
www.pcgamemods.com) , or
www.lucasfiles.com) for a jawa skin mod and download it. Change the name of the .pk3 file to whatever you want (name starting with B-Z), then replace or insert your new files into that mod. The mod should already have the correct internal directory structure, which you must maintain. Then put your new .pk3 in your base folder and it will override the standard files.
Simply remove the mod .pk3 file from the base folder to return to normal.