If you've assigned the wrong currency to a card
If you discover the problem before you've recorded any transactions using the card, simply return to the Card Information window and select the correct currency.
If you've begun recording transactions using the card, you have two options: You can delete every transaction that uses the card, then return to the Card Information window and select the correct currency. Or you can create a second card and assign it the correct currency, and begin using that card for new transactions.
Although it's generally easier to create a second card in this case, doing so will make it more difficult for you to assess the card's long-term sales or purchases history.