To add the new tracking number you still need to generate the original prepaid label (which you have done) then go to the More Actions dropdown menu or via View Order Details > Postage > Track Package > dropdown arrow, and change the number. That needs 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 cannot manage to do the above the payment will be released anyway in 14 days.