ROOT user on Ubuntu server

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