Ssh-keygen -p

    -p      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.

See also

• ssh-keygen [ -R, -t | -p | -i | -y | -f | ~/.ssh/id_rsa | --help ] ~/.ssh/known_hosts