This mod replaces the Super Battle Droid with the B1 Battle Droid for the Assault class.
To install this mod, you will need to head over to:
Program Files (x86)/Steam/steamapps/common/Star Wars Battlefront II/GameData/DATA/_LVL_PC/SIDE
Once you are there, replace the original "cis.lvl" file with the new one. Make sure to copy the original file in case you don't want to use this mod anymore.
If you do not have the game on Steam, the directory should be:
Program Files/LucasArts/Star Wars Battlefront 2/GameData/DATA/_LVL_PC/SIDE