Go to the menu screen to see a list of your current party and other cards that are there for backup. On your line up, tap the card that you wish to improve from enhancing. There are three ways to boost your card. The first is to drag an unneeded character card over your highlighted card. This is sacrifices the least important cards to improve your higher priority card. Sacrificing a card will let you gain experience points as well as increasing all your stats. Another way of improving your cards is to equip them with gear cards and consumable cards. Consumable cards have limited uses and will eventually disappear. To activate one, drag it onto the card you wish to improve. A gear card once equipped cannot be separated from the card that it was bound to. A card can only be equipped by only one gear card.