Once you’ve been dealt your starting Power-ups you can buy different Power-ups to replace those at any time! Just click on the yellow Power-up Store button:

There you’ll see your current Power-ups in the Power-up Slots on top. Power-ups available for purchase are pictured below. Tap on the Information Icon on any Power-up to learn more about what it does. To select a new Power-up just tap on it and then tap on the Power-up Slot you want to assign it to. If you’re already played one of your Power-ups, you won’t be able to use that slot for the remainder of the game.