When you purchase the necessary aero parts from GT Auto, you tune it in the menu others have mentioned. On most cars, you can only adjust the rear downforce.
It has been brought to my attention that some cars have a static value of positive front downforce, but it cannot be adjusted. Example: The McLaren F1 has a static value of +20 non-adjustable front downforce after you purchase aero parts. The GT-R SPECV has +2.