You can either cancel/refund the transaction with the buyer's agreement (via the dropdown actions menu arrow for the item on your orders list) and relist with RM postage for the buyer to buy and pay again.
Or buy and pay for the RM postage yourself. To add that tracking number you still need to generate the original prepaid label (although it will remain unused). Then add the new tracking via View Order Details > Postage > Track Package > dropdown arrow > Add Tracking. That needs to be done on a desktop browser.
Afterwards 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.
