This guide shows how to manually record a customer refund, however, for those explaining the refund from an imported bank statement, just follow steps 4 & 5.
Navigate to Money > Bank accounts.
Click on the Money out link beside the appropriate bank account.
Fill in the details of the Money out form and hit the Explain money out button.
On the Explain Transactions page select the Refund tab.
Enter the details of the refund and click the Add new transaction button.