As you progress through the Campaign and fill your collection, there may be times when opening a reward yields a card of which you already have the maximum number of copies you can use in a deck. Whenever this happens, you will store that duplicate card in your account, and will be able to use it to visually upgrade your collection! This upgrade will not affect the card’s power at the hour of battle, but it will make it pop up and stand out among its peers on the battlefield. Not only that, upgrading cards from the same army will allow you to increase that army’s Forge level, and get you amazing rewards!

To upgrade a card, you'll need to spend the required amount of duplicate cards, plus the required amount of Crystals. You can do so from the close-up view of a card in your Collection, either from the Cards Collection or the Deck Editor.