Edit /etc/default/login with your editor (probably vi). Navigate to the line which reads
# If CONSOLE is set, root can only login on that device. # Comment this line out to allow remote login by root. # CONSOLE=/dev/console
and comment it out.
Of course, don’t forget to change /etc/ssh/sshd_config:
# Are root logins permitted using sshd. # Note that sshd uses pam_authenticate(3PAM) so the root (or any other) user # maybe denied access by a PAM module regardless of this setting. # Valid options are yes, without-password, no. PermitRootLogin yes