You have ebay's simple delivery on your listing so you won't be able to cancel the label. You can either cancel the transaction via the dropdown arrow for the item on your orders list (a refund will be issued) and relist with correct postage for the buyer to buy and pay again. Let them know first.
Or buy the new label out of your own pocket. If you do the latter you need to contact ebay and ask them to change the tracking number. Or you can leave it and wait 14 days for the money to be released.
If you want to contact ebay you can use callback on here:
https://www.ebay.co.uk/help/eua?id=5275&mkevt=1&mkpid