To upgrade a card, you must have acquired the minimum number of duplicates of that card. If you have the necessary number of cards it will also cost gold for offensive units, and wood for defensive structures to upgrade.
You can upgrade your cards:
- Tap the CARDS button in your city
- From your card collection you can see which cards are available to upgrade
- You can see the requirements on the card, or within the card info screen by tapping on the card