To generate a new label for the shipment with a different service you will want to follow the below steps.
Head to Shipping History > Search for your shipment using the tracking number or order number for the shipment.
Once you find the shipment click on the blue plus icon under actions > Click on "Reprocess Shipment" > make sure your "How to load shipment" is set to "Reprocess Original" > Click the "yes" button for "Would you like to void this shipment before reprocessing it?"
Then you will be brought back to the shipping page, and you can alter the shipping information that you are needing changed for the shipment and click process to get the new label.
Note: That this will generate a whole new label with a new tracking number as well.