If you want to change the Truck you're driving you can simply Tap it to switch to it. If you have more than 3 Trucks then other trucks are in a different Garage. You can access them by tapping the Garage button, which are located at the left side of the screen.

Your Trucks are grouped by 3 in Garages. Swipe between your Garages to find the Truck you wish to choose and Tap it twice to select it. You can also tap the arrows to switch between Garages