As you progress through the game you will unlock new Racers. You can cycle through your available Racers by tapping on the arrows under your currently selected Racer.
You can also purchase and swap between special Racers by tapping the button to the left of your current Racer.
From this screen you can cycle through the available Racers using the same arrows as in the previous screen, and either buy a new Racer or equip any previously purchased Racer. Simply tap the Equip button under any previously purchased Special Racer to use them. Tap OK to return to the main Racer screen.