You should be able to change size and/or weight.   Click on Get Prepaid Label and under the heading Package details click on the pencil icon and you should get: "No extra charge. Changing your package size won’t cost you anything."   Then use the dropdown menu to change weight.

The alternative is to 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.