There is 18 different cars in the game. Prototypes excluded from this achievement while that cars can't, so only need 15 cars to upgrading.
While you playing Events is Carrier mode can earn Rating points for prize-winning positions. This rating points can use to upgrade the cars.
Need collecting 8750 points total to upgrade every cars what need. After have enough points you can immediately spend it on any available upgrades in you Garage or before Events start on Car selecting screen.