Login to the pi (user: pi password: raspberry):-

login as: pi
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Optionally Change the pi user password:-

pi@realtimepi:~ $ passwd
Changing password for pi.
(current) UNIX password:
Enter new UNIX password:
Retype new UNIX password:
passwd: password updated successfully

Set a root password:-

pi@realtimepi:~ $ sudo su
root@realtimepi:/home/pi# passwd
Enter new UNIX password:
Retype new UNIX password:
passwd: password updated successfully

Permit Root login over ssh

Edit /etc/ssh/sshd_config Change:-

#PermitRootLogin prohibit-password

to

PermitRootLogin yes

Done.