On the customer account page you can view a list of past transaction by clicking the transactions tab. You can refund any of these transactions by clicking the Refund button next to the transaction, then:
Enter the amount that you wish to refund the customer and click Refund. The refunded amount will be issued to the customer's credit card and debited from the credit card associated with your account. (See Enabling Refunds)
If the underlying transaction is not yet settled, you are able to void the transaction instead. The charge will not be added to the customer's payment.
Remember: If you have not yet enabled refunds by adding a credit card associated with your account, you will not be able to issue refunds.