To login server as root, we should set a password to root user.
sudo passwd root
After that root access should be permitted. To make this, we will modify sshd_config file.
sudo nano /etc/ssh/sshd_config
Find the "PermitRootLogin" line and replace it with "PermitRootLogin yes".
Restart sshd service now to affect changes (for Ubuntu: service ssh restart).
sudo service sshd restart
From now on you can access your server via SSH as root with username and password.
