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.
Re-shipping on a different carrier
The same Reprocess Shipment action moves a shipment to a different carrier, not just a different service. This is the standard fix when a carrier is discontinued or goes out of business and your parcels come back, or when the wrong carrier was used.
Reprocess the shipment as described above, and when the original shipment data reloads, change the carrier before you process. A new label and a new tracking number are created, the same as any reprocess.
If the old carrier is being retired for good: email support@shipstore.com and ask for the retired carrier account to be removed, so it stops appearing as a choice and nobody ships on it by mistake.
For a large number of returned shipments, reprocess them one at a time as above; there is no bulk reprocess. If the volume is high, email support@shipstore.com with the carrier and roughly how many shipments are affected.