In the Menu Bar on the left, go to Reports>Invoices. Click on the invoice number, a new tab opens; scroll and locate Refund. You can select a session or a retail item for which you want to generate the refund in the pop-up window. Click on Refund, and the amount gets refunded to the customer.
Yes, you can refund in full or make a partial refund to customers who have paid against their orders.