To change tracking you have to generate the original prepaid label. It sounds like you've done that so go to View Order Details > Postage > Track Package > dropdown arrow and see if you can change the tracking. That has to be done on a desktop browser.
If you can't do it contact ebay and ask them to do it. You can request a callback on here:
https://www.ebay.co.uk/help/eua?id=5275&mkevt=1&mkpid
Afterwards if you paid for the original label you can cancel it 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 don't manage to do the above the payment will be released anyway in 14 days although you won't have proof of delivery.
For future reference eBay's Simple Delivery covers items up to £750. In the case of loss or damage in transit ebay refund the buyer, the seller keeps the payment.
