The tracking for the label that was used has to be added onto ebay before you can cancel the original one to get a refund. Do that via View Order Details > Postage > Track Package > dropdown arrow > Add Tracking on a desktop browser.
If you paid for the original label the listing would have had "Free postage" on it and you can cancel it here:
https://www.ebay.co.uk/claims/simple-delivery/refund/seller
If the buyer paid for it (if the cost was displayed on the listing that means the buyer paid) it will be automatically refunded to them in 30 days.