You could 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 correct postage yourself. However you cannot add a new tracking number. All you can do is message the buyer with the new tracking details. You would still be able to add the new number to a "Not received" case if the buyer opened one though.