RHA's upgrade system is pretty simple, but there are some details and stats in here that's different from standard racing games.
Garage Interface Edit
This is the Garage in RHA, it shows all of the cars available in the game, including the ones you currently own. The 4 stats are in order from top to bottom: Speed Boost, Acceleration, Health and Nitro.
There is one stats here that needs some explanation: Speed Boost, this stat affects how fast your car accelerates when it's following behind other cars.
Car Upgrade Interface Edit
Pretty straight forward, left, right arrow to add or subtract points and tap the "Upgrade" button to finish.
The TP (Tech Points) determines your total upgrade points, though it is expandable by using Diamonds in the "TP Slot Expansion".
I'll put up some more info on how much weight each point is once I have those info.