This guide shows how to manually record a supplier 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 in link beside the appropriate bank account.
Fill in the details of the money in form.
On the Explain Transactions page select the Refund tab.
Enter the details of the refund and click the Add new transaction button.
Alternatively, if the supplier has any outstanding negative bills they will appear in the drop down list below the contact drop down field. Click on the add transaction button beside the appropriate bill.