Coins can be used to buy boosters from the in-game shop.
To use them just tap the 'Shop' tag which is shown at the top left hand side of the pre-level 'goals' screen. This is shown just after you've selected the level you're about to play.
The game will show the stars required and the goals for the level, with the 'Shop' tag at the top of the screen.
Important note: You can only buy boosters in exchange for Coins before a level begins. Once you are in a level, boosters will be unavailable, so come prepared!
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