Tap on your car in the main menu to enter the Garage. To upgrade your car, you will need gold and spare parts. Both can be found in the supply boxes that you get by winning quick fights and being promoted to the next stage or league.
There are 4 types of spare parts:
- Chassis
- Wheels
- Weapons
- Gadgets
The chassis is the "body" of your car. It has several randomly placed slots where you can install other spare parts. Each part has its own type of slot. Weapon slots are hexagonal, while gadget slots are square. You can't put a part in a slot designed for a different type of part.
To install or change the part, simply move it into the needed slot.
You can also upgrade the parts that you already have. To do this, you need to fuse the unused parts into them. To upgrade a part, first tap on it. You will see its full characteristics in the window that will open on the left side of the screen. While keeping this window open, scroll through your inventory until you find the unused part that you are ready to spare. Drag it into the window with the part you want to upgrade and fuse them together. This will increase the characteristics of the upgraded part. The unused part will be destroyed. You can fuse different type of parts with each other. For example, you can upgrade a wheel by fusing a rocket launcher into it.

Be aware that each fuse will cost you some gold coins. The price depends on the class of both parts: the better each part is, the more gold you'll pay for a fuse.

And don't forget the stickers! Make your car more stylish by applying them to your chassis.