The following procedure does not change the private key. The procedure changes the authentication mechanism for the private key, the passphrase. For more information, see the ssh-keygen(1) man page.
Change your passphrase.
Type the ssh-keygen command with the -p option, and answer the prompts.
myLocalHost% ssh-keygen -p Enter file which contains the private key (/home/jdoe/.ssh/id_rsa):<Press Return> Enter passphrase (empty for no passphrase): <Type passphrase> Enter same passphrase again: <Type passphrase>
where -p requests changing the passphrase of a private key file.