Simple Delivery labels can't be cancelled. The only way of changing it is to cancel the transaction and relist, making sure the weight bracket is correct, for the buyer to buy and pay again.
If you prefer to buy the correct postage with this transaction you would pay for that yourself. If the first label is not used the buyer will get a refund on that.
To change the tracking you can contact ebay and ask them to change it. You can use callback on here:
https://www.ebay.co.uk/help/eua?id=5275&mkevt=1&mkpid
