You must constantly improve your Truck to win Races.

To do this, tap the Upgrade button at the bottom-center of the screen in the Main Menu.

Note that you will be upgrading the selected Truck.

Swipe through the different upgrades at the bottom of the screen.

Upgrade by tapping the upgrade button. You can spend either silver or gold.

The best advice is keep Stages of upgrades on the same level. You don't want to upgrade, for example, Engine to Stage 5 before upgrading your Tires, because you will have a bad grip with the road, making your Stage 5 engine ineffective.