Once you have parts tap on the car of your choice to bring up its menu. At the bottom tap on the “Upgrade” button which will upgrade in the number of increments shown on the bottom of the button. Note along the top you will see a gold star with a number. This shows the level of your car. You can upgrade a car until it is at a maximum level of ten. Each time you upgrade a level note that the amount of money the car earns for you will increase.