Thank you!
Changing the car is really simple, the car is made in a 3D out of 5 components:
- CarBody
- Tire_frontleft
- Tire_frontright
- Tire_rearleft
- Tire_rearright
Example car mesh download link: https://www.lossy.studio/downloads/SK_Car.FBX
When you make your car in a 3D program, make sure you connect the body parts, and have the wheel parts separate. Also make sure the axis is in the centre of there corresponding part:
Export the car as a .FBX file (or use the example file above) and place it on a good location in your project structure. In your unreal project go to: “PolyCar_Pack -> PolyCar -> Main” and open “car_mesh”. Above in this window click on “Reimport Mesh” and select the example car mesh file (or any other car .FBX file).
Now the project car should change into your .FBX car. Close the window and test it out. If you want to edit the car mesh, make the changes in your 3D program, overwrite your old .FBX (location you saved it before) and “Reimport Mesh” again.
And you are good to go! 
You can of course change for example some suspension settings or engine settings to change the handling for your car.
Greetings! 