By upgrading your crew members, you increase their impact on a car's abilities. To upgrade one of the crew members, tap the "Crew" button on the main screen. Choose the member you want to upgrade from the list.
When you tap the "Fuse" button, a window appears. In the left part of the window you can see the chosen member's current level and the amount of experience needed to upgrade him. In the right part of the window is a list of crew members that can be used for upgrading. The amount of experience the target crew member will get depends on his tier and the tier of the members you use for upgrading.
When your crew member reaches levels 5, 10 and 15, you need to evolve him.
To do this you need to use two crew members of the max level for this tier.
You can evolve the crew member for free ("Random") - in this case one of the two crew members will remain, chosen at random. When you evolve crew members for crystals ("Specific"), you can choose which one will remain.
After evolving your crew member, you can continue to upgrade him.