How do I edit Vehicles?

I just watched a video of someone building the Gavril Barstow from the chassis up. He added doors, changed wheels, etc. When I tried this the only options it gave me to change were the license plate design, paint job, and body (normal body or convertible body). To be honest I am not the best at this game. I have fiddled around with the settings but cannot find an answer. If someone could tell me what I am doing wrong it would be greatly appreciated. I am new to this website and appreciate any advice you have. Thank You!

 

click on the little arrow

 

Thank you so much for your help!