If you have paid invoices and you have decided to issue refunds, you can accomplish this directly from the Attendee Purchases tab of the event. Click the Actions button in the Actions column, and select "Create Refund". Add a note to the refund, and if needed, you can modify the amount to refund either a portion of the payment, or the full payment. Credit card refunds will be automatically processed through your GZ payment processor, but if you've already processed a credit card refund outside of GrowthZone directly through your processor, you can select the "Refund Already Issued by Processor" option. This will record the refund in the GrowthZone software, without a refund being issued back to the purchaser on the processor side.
Refunds for check payments will need to be manually managed according to your business processes for cutting checks/returning funds. You may also issue a credit toward a future event or invoice.
More details on issuing refunds can be found here.