Once your reservation is confirmed, you cannot change it. However, you may cancel it. For further information, please read the article "I want to cancel my shipment, how do I get a refund?". If you choose to cancel your reservation, you'll need to place a new order, which will result in new labels being added to your shipment.
So make sure to check first that you are able to print and place these new labels on your shipment before making any changes.