You can't change a Simple Delivery label. The cost of an unused (ie. unscanned) label will be returned to the buyer, assuming they paid for it, within 30 days.
For your item you can cancel/refund the transaction with the buyer's agreement (via the dropdown menu arrow for the item on your orders list) and relist with correct postage for the buyer to buy and pay again.
The alternative would be to buy and pay for the postage yourself outside of ebay.