You have always two marbles to choose from - the one is inside Shooter's mouth and the other is peaking through a hole on its back. If you play on mobile, in order to swipe them you need to tap the Shooter’s shell. On PC, you can switch marbles by clicking the right click or pressing the space button.

 

 

On some levels, you can also change the Shooter’s position. All you need to do is to tap/click on the platform and the Shooter will appear there.