You can delete an item from an order by clicking on the order number, and scrolling down to the line items. Each line item should have an X next to it, allowing you to delete the item. However, if there is only one item in the order, the X will not be present. Thus if you are trading one item for another, add the new item to the order first (click Line Items / Add). Then you will be able to delete the other item from the order.