If you didn't have Simple Delivery on your listing find the label on here:
https://www.ebay.co.uk/ship/lmng/labels-history/seller-hub
On the left of sales record there's a dropdown menu under "Actions". Click on that then click on "Void Label".
If you did have SD the label cannot be cancelled. You'd have to buy another one and then see if you can add the new tracking to the order by going to the Order Details > Postage > dropdown menu (beside track package) > Add Tracking > + Additional Tracking Number.
If not contact ebay and ask them to do it. You can use callback on here:
https://www.ebay.co.uk/help/eua?id=5275&mkevt=1&mkpid