Its a rather complicated process and it involves a lot of editing of code which trust me takes forever to figure out what words to change, not advised for a beginner
In what way do you want to change it? Just to select from the ingame parts, or edit the code yourself?
There is no way to on these vehicles Spoiler: List Ibishu 200BX Gavril Barstow Gavril Bluebuck Civetta Bolide Ibishu Covet Gavril Grand Marshall Bruckell LeGran Ibishu Miramar Bruckell Moonhawk 1996-2000 Ibishu Pessima Autobello Piccolina Ibishu Pigeon Burnside '53 Special Wentward DT40L Gavril T-Series That leaves all the ETKs, all the Hirochis, the Ibishu Hopper, the 1998-1992 Ibishu Pessima, and the Gavril H/D-Series/Roamer. Step one is go to the vehicle parts selector from the game menu (Green circle). Then go to engine --> transmission --> transfer case and select the one you want (red arrow). Watch out for differentials and driveshafts (blue arrow), make sure you have something appropriate selected. The 1998-1992 Ibishu Pessima and both Hirochis have a differential under the engine --> transmission tab.