It is not advisable to use password based login for ssh any more. One of the most secure ways of remote login through ssh is to use public key authentication. But in order to do that you have to generate both private and public keys using ssh-keygen, for RSA the default file names would be id_rsa (private key) and id_rsa.pub (public key). Since you have already generate those keys with a passphrase and want to change the passphrase now, execute the following command:
$ ssh-keygen -f id_rsa -p
type your new passphrase, you are done!