How do you decrease the polys on a vehicle model

[Maryland Modifications 33]

I've made a vehicle model and it crashes my game, I know it's to big so I'm wondering how you decrease the number of polys in a vehicle file

[EyON 27]

You cannot, except by re-doing the vehicle.

A polygon is a shape that represents a little part of the vehicle. You can't lower the number in a file. 

You would have to make it less detailed than it is