If you want return tickets from an Order, you will need to create a new Order with a refund transaction.
When you click the Refund button on an Order page, the current tickets will be added to the basket in Box Office as items to cancel and refund (they will appear in red). This means you have created a refund basket.
You can then add or remove items from this basket if you wish, by going through the normal Box Office flow.
Once the basket has the items that you want to refund, continue into the payment page, select the Payment Method to refund to and complete the refund transaction - a new Order will then be created that is linked to the original order.