You do need to open the label as the buyer's correct address will be there. You can't cancel it.
Ebay will charge you for the label but you can claim the cost back - see the link below.
You won't be able to change the tracking but should probably message the buyer with the new tracking number.
f you selected ‘Seller pays’ and offered free postage on your listing, you'll need to fill out the Request a refund for Simple Delivery form for the label that wasn’t used. Make sure to send your request within 14 days of your item selling
"There are two ways to be fooled. One is to believe what isn't true; the other is to refuse to believe what is true.”
Søren Kierkegaard, Danish philosopher (1813 - 1855)