You have always two marbles to choose from - the one is inside Woka’s mouth and the other is peaking through a hole in its back. If you play on mobile, in order to swipe them you need to tap the Woka’s shell. On PC, you can switch marbles by clicking the right click.
On some levels, you can change the Woka’s position. All you need to do is to tap/click on the platform and Woka will appear there.