How to sign existing transaction?

[madmadmax]

How can I sign an existing transaction with zero confirmations with a different input easily?

[DannyHamilton]

You'll need to use rawtransactions.  As far as I know, there are no current wallets that will let you double spend an input intentionally from the user interface.

If you are using Bitcoin-Qt, you can use createrawtransaction, then use signrawtransaction to sign it, then use sendrawtransaction to broadcast it.

However, you'll need to make sure that your current wallet is not continuing to broadcast the old transaction, and then you'll need to wait a few days for all peers to remove the unconfirmed transaction from their memory pool.  Otherwise, the peers will all refuse to relay your double spend transaction.