To upgrade a car, go to the Garage and tap the Upgrades icon. Here, you can upgrade each stat (Top Speed, Acceleration, Handling and Nitro) by paying using Credits you earn in the game. After upgrading the same stat several times, you can also plug some Import Parts into it to improve the stat a bit more. You also need to gather more Blueprints to Star Up your car and unlock more Upgrade Stages for your stats.