You can upgrade each truck attribute in the truck selection menu by pressing the truck icon in the top bar. Here’s how it works:

Upgrades cost truck coins.

You earn these by collecting them on the track.

Upgrade coins earned are specific to each truck.

For example, if you want to upgrade the Rip Rod truck then you can only use Rip Rod Truck Coins, which will be any coins you collect whilst driving the Rip Rod truck. Alternatively, you can pay for the upgrade or part of the upgrade with Gems. Gems are more flexible and so can be used to upgrade any truck, not just a specific one.