You must have "Full Access" or "Limited Access - Financials" permissions to refund invoices.
You can issue a refund for an invoice from Invoices or the Payout Report.
From the left navigation menu, click Financials > Invoicing.
Find the invoice you'd like refunded and click the Sale ID.
In the top-right, click Issue Refund.
Choose the payment to refund and enter the amount to refund.
Select Yes or No for Do you still need to collect this money in the future?
If No, add any notes and click Refund.
If Yes, add any notes and click Next.
Enter the date to recollect the refunded amount.
Click Refund.