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 correct postage for the buyer to buy and pay again.
Or you could buy the correct postage yourself. 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 automatically in 30 days. See if you can add the new tracking number via the dropdown menu (on a desktop browser). If not the payment will be released anyway in 14 days.