Issuing a refund
Navigate to your desired Project and select "Issue Refund". You will also see options to Record Offline Payment or Capture Payment.
Enter the amount to refund, select the Origin, and if you're refunding and Online payment method where your client paid the fees, you'll be able to select if you'd like to refund these fees, too.
You will see the changes made on the Billing page under Online Payments.