Here is an animation of how you can do it in your Back Office.
Go into transactions > click on the transaction in question > on the top right of the screen click on "refund" > select reason > select the type of operation (refund only, expire and refund)
Here is an animation of how you can do it.
Please use the comments section below to ask any additional information.
You can also contact us at clientservices@vendoservices.com