Pretty easy! Go to the Gallery (the fourth icon at the bottom tabs), click on a card you’ve already unlocked (you can sort by All, Equipped, Rarity, or Unlocked). A small popup will open below the card, click the ‘Equip’ button. Click it again if you want to unequip. Also, you can equip up to five cards, but you need to unlock that ability in the Upgrade menu first!