If you didn't use the Simple Delivery label to add the new tracking number you still need to generate the original label. Then go to the More Actions dropdown menu to add the new tracking or via View Order Details > Postage > Track Package > dropdown arrow. That has to be done on a desktop browser.
After that you should be able to cancel the original label here:
https://www.ebay.co.uk/claims/simple-delivery/refund/seller
Or if the buyer paid for it it will be refunded to them automatically in 30 days.
Even if you can't do the above the payment will be released anyway in 14 days.
