change existing ssh passphrase

$ ssh-keygen -p -f ~/.ssh/id_rsa

Enter old passphrase:
Key has comment 'user@hostname'
Enter new passphrase (empty for no passphrase):
Enter same passphrase again:
Your identification has been saved with the new passphrase.

https://serverfault.com/questions/50775/how-do-i-change-my-private-key-passphrase