How to edit a mesh/car texture

Hey, I hope this is the right category. Anyways, Roblox released some new cars today. And on that occasion, I was interested in using them. However, I want to change the texture the cars have. And I am sure how to do that. Basically, I want to turn the car into a police car, which means that I need some sort of UV map to edit and then put onto the car again.

I also found out that the cars are not 1 part. So the doors are their own mesh, and not a part of, say the hood.

My question is basically the easiest way to edit and texture the mesh into a police car.
I have blender

A very vague example:

Also, please read the post below (the end of it) as I wrote a little more there also:

Thank you very much for help!!