Once you have maxed a stage in your car upgrade, you have the option to take your upgrade up to Stage 2, and then Stage 3 by fusing 3 cars with your own.

Fusing requires you to have 3 Cars of equal or one less rarity to fuse with (a good way to make use of those Cars you don't want!), for example you have a Rare Card which means you can Fuse with other Rares as well as Uncommon Cards.

Here's a list of how to know which card is what rarity:

Colour Grey = Common (F)

Colour Green = Uncommon (E)

Colour Blue = Rare (D)

Colour Yellow = Super Rare (C)

Colour Red = Ultra Rare (B)

Colour Purple = Epic (A)

Colour Gold = Legendary (S)