This post is older than a year. Consider some information might not be accurate anymore.
Existing (private) ssh keys may have a password. If you want to alter the password, the ssh-keygen command will reassign the pass phrase to the existing private key.
# RSA algorithm
ssh-keygen -p -f ~/.ssh/id_rsa
# DSA algorithm
ssh-keygen -p -f ~/.ssh/id_dsa
