First for all edit ssh config file:
sudo nano /etc/ssh/sshd_config
Uncomment this line:
PermitRootLogin yes
Then restart ssh service:
systemctl restart sshd
for 24.04
sudo systemctl daemon-reload
sudo systemctl restart ssh.socket
Next step, create password for root user:
sudo passwd root
After that login as root and (optional) delete existing not root user:
killall -9 -u {username}
deluser –remove-home username