You have simple delivery on your listing so you should have used the postage label supplied by ebay and the tracking would be registered automatically.
If you bought your own postage you can't change the tracking number but if you paid for the label on ebay as well 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.
The payment will be released anyway in 14 days.
