To upgrade cats, save experience points and use them to level up. You will not be able to see the specific improvements, but will notice them during battles. To earn more experience points, upgrade the Cat Study. This will allow you to earn more experience in each level. Upgrading the Cat Cannon will increase its range, damage and charge speed. You can use it after completing its charge to hit every cat that is behind the pink dotted line. To be able to play longer, upgrade Cat Energy. Cents will allow you to summon more cats during battles. To get more Cents faster, upgrade the Generator.