Upgrading your Pops! increases their Level and makes their Super more effective, which can lead to higher scores and more Power-Ups and Mystery Boxes collected. Pops! can be upgraded on the collection screen. Once you've earned enough Pops! through Chests or Mystery Boxes, select that Pop! and tap the UPGRADE button. It will cost you Coins to upgrade your Pops!. If you're out of Coins, you can earn more through gameplay or convert Gems to Coins in the Shop.