Adding or changing a passphrase for an existing private key
You can change the passphrase for an existing private key without regenerating the keypair by typing the following command:
$ ssh-keygen -p -f ~/.ssh/id_rsa
> Enter old passphrase: [Type old passphrase]
> Key has comment 'email@example.com'
> Enter new passphrase (empty for no passphrase): [Type new passphrase]
> Enter same passphrase again: [Repeat the new passphrase]
> Your identification has been saved with the new passphrase.
If your key already has a passphrase, you will be prompted to enter it before you can change to a new passphrase.