Best to cancel and relist. Go to the dropdown arrow alongside the item on your sold page > Cancel order. If buyer doesn't reply use the reason "Something wrong with buyer's address", nearest one to cover general problems. A refund will be issued in the process. Let buyer know what you're doing so they can buy again if they want to.
The alternative would be to buy the postage yourself and try to change the tracking number on ebay via the More Actions dropdown menu or View Order Details > Postage > Track Package > dropdown arrow. That needs to be done on a desktop browser. If not the payment will be released anyway in 14 days but you won't have proof of delivery.
Then if you paid for the original label you can cancel it here:
https://www.ebay.co.uk/claims/simple-delivery/refund/seller
If the buyer paid for it it will be refunded to them automatically in 30 days.