Change a Passphrase With ssh-keygen command

The -p option requests changing the passphrase of a private key file instead of creating a new private key. The program will prompt for the file containing the private key, for the old passphrase, and twice for the new passphrase. Use -f {filename} option to specifies the filename of the key file. For example, change directory to .ssh:

$ cd .ssh
To change RSA passphrase, enter:
$ ssh-keygen -f id_rsa -p
To change DSA passphrase, enter:
$ ssh-keygen -f id_dsa -p