To refund a credit payment click on the Payment line item in the list of Invoices on the patient's Accounts screen, then click the Refund button.
If you wish to refund using Tyro click the Refund with Tyro button, change the amount if necessary, then click Send to Tyro.
If not, simply enter the payment method then click Proceed.
To reverse a refund delete it using the Delete process outlined here.