If you've bought your own label to add the new tracking number and mark as dispatched you still need to generate the original prepaid label.
Then go to the More Actions dropdown menu or View Order Details > Postage > Track Package > dropdown arrow and change the tracking. That needs to be done on a desktop browser.
After that you can 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 change the tracking number the payment will be released anyway in 14 days.
@tclind67