Open the sale (from Transactions or Orders in the terminal, or from Admin > Customer Orders) and choose to refund it. You can refund the full amount, a custom amount, or a partial line (a specific meal, extra, product, expense, gift card, or custom charge) and pick which method the refund goes back to (the original payment method, or account balance if the customer has one). A confirmation summary with the amount and method is shown before it's processed.