To fuse a car:

1. Go to your garage
2. Select the car you want to upgrade
3. Click the Fuse button
4. Select the cars you want to fuse into your car (be careful not to fuse cars you want to use later on!)
5. When you have sufficient credits loaded, hit the Fuse button.

You can fuse a car a maximum of 2 times

Depending on the tier, the car starts with a different star level

For example, Tier D cars start at 1 star and can be fused to 3 stars

Tier S cars start at 4 stars and can be fused twice to reach 6 stars

Tier D = Tier D cars only
Tier C = Tier D and Tier C
Tier B = Tier D, C, and B
Tier A = Tier D, C, B and A
Tier S = Tier D, C, B, A and S