You can't cancel or upgrade a Simple Delivery label. The correct thing to do would be to cancel/refund the transaction via the dropdown arrow for the item on your orders list, and relist with correct postage for the buyer to buy and pay again.
An alternative would be to buy a new label out of your own pocket. The original label will mark the item as dispatched but of course the new tracking won't show deivery on ebay so you'd have to wait 14 days for the payment to be released.
A refund for the original unused label will automatically be refunded within 30 days.
