Instructions
Visit one of the three car modification shops in San Andreas. There is one in each major area of the city: Los Santos, San Fierro and Las Venturas. You will need to enter the modification shop with a four-wheeled car or truck; bikes or specialty vehicles, such as golf carts, cannot be customized.
Select the "Wheels" option in the modification menu. This menu will appear once you enter the modification shop with a car (or truck).
Select the wheels you want to add to your car, then choose to pay the amount required for the wheels. The new wheels, complete with new rims, will be added to your car.