I update all of my passwords at least every 72 days. Since I use git via SSH I wanted to update my private ssh rsa key also. To update the password for the key at ~/.ssh/id_rsa use ssh-keygen as follows:

# -p to request password change, -f to specify the filename
$ ssh-keygen -p -f ~/.ssh/id_rsa

Follow the prompts (old password, new password) to update the password.

Also see How to update GnuPG gpg key passwords