Note that double spending has a high likelyhood of failure, and the initial transaction may be confirmed. Help
Double Spend Help
This tool allows you to generate two transactions using the same Bitcoins.
The first transaction will have a very low fee, and will only be accepted by a few nodes, but after some time it should make its way to the recipient.
The second transaction will have a generous fee and will be broadcast more easily to many nodes.
Publish the first transaction and wait for the recipient to receive the unconfirmed transaction. You can then broadcast the second transaction and it should reach miners first as long as the first transaction has not yet been confirmed.