Just follow the steps below:

  1. Open the terminal and type:
    sudo passwd root
  2. When you see the prompt that says "Enter new UNIX password", enter the password you want for the root user and confirm it.
  3. E the “Log-In Window” settings under the System > Administration menu you to do this easier. Under the “Security” tab enable “Allow local system administrator log in”.
  4. Exit gedit, logout and then reboot.