1.  Click "Pages" in the top menu.

2.  Find the page on which the transaction was made and click the "open" button on the right.

3.  In the left menu, select "Transactions"

4.  You will now see a list of transactions made on this form.  Look for the one you want to refund.  

Use the blue arrow buttons at the bottom to move through the list.

5.  Once you have found the transaction that needs to be refunded, click the blue pencil icon to the right of it.

6.  Now you will see all the details for the transaction.  Scroll down and click the "Tasks" button.

7.  Select "Refund Transaction" from the menu that appears.

8.  You will be prompted to confirm the refund.  Select OK to complete the refund.

A new transaction with a negative amount of the refunded transaction will be created, and you will be returned to the list of transactions.