To mark it as dispatched and add the new tracking number you have to generate the original prepaid label. Then you can change the tracking via 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.
If you cannot change the tracking the payment will be released anyway in 14 days.
