By upgrading your cars, you make them more effective, giving you a significant advantage in battle. To upgrade your car, you need car parts. You can get them by opening chests or producing them from scrap. The amount of parts needed to make upgrades is shown on the car's card in the garage.
Every upgrade increases one of the car's parameters (randomly).
You can redistribute the upgrades if you don't like the current build.
Tap the "Modify stats" button in garage.
You can redistribute one grade point randomly, or all points manually.