When you accept or mark an expense as refunded, it moves to the Completed tab and it's not possible to edit it anymore.
You can however click on the "Revert decision" button (accessible from the side menu when you click on an expense from the Completed tab). This will move the expense back to the Upcoming tab and become editable again. You can then accept or reject it again. If the expense was previously exported to Xero, we will update it's data so to avoid duplicates.