1. How to Enable Root Account in Ubuntu?
To Access/Enable the root user account run the following command and enter the password you set initially for your user (sudo user).
$ sudo -i

2. How to Change Root Password in Ubuntu?
You can change root password with sudo passwd root command as shown below.
$ sudo passwd root Enter new UNIX password: Retype new UNIX password: passwd: password updated successfully

3. How to Disable Root Access in Ubuntu?
If you wish to disable root account login, run the command below to set the password to expire.
$ sudo passwd -l root

4. How to enable ssh root access in Ubuntu?
Login to your server as root.
As the root user, edit the sshd_config file found in /etc/ssh/sshd_config:
vim /etc/ssh/sshd_config
Add the following line to the file, you can add it anywhere but it’s good practice to find the block about authentication and add it there.
PermitRootLogin yes
Save and exit the file.
Restart the SSH server:
systemctl restart sshd
or
service sshd restart
And that’s it! With the new line added and the SSH server restarted, you can now connect via the root user.
In this instance, you are going to be able to login as the root user utilizing either the password or an ssh key.
When using SSH Keys, you can set the PermitRootLogin value to `without-password` instead of yes. To accomplish this, simply modify the following information noted in step 2 above instead:
PermitRootLogin without-password
This process should work on almost any version of Linux server that the sshd service is installed. If you are using a cPanel server though you can easily control this setting from the WHM interface. In these cases, it’s recommended to modify this setting from your control panel interface.