0 x00 Set password for root user

dora@ubuntu:~$ sudo passwd root
[ sudo] password for dora:
Enter newUNIX password:
Retype newUNIX password:
passwd: password updated successfully

0 x01 Modify 50-ubuntu.conf configuration file

sudo gedit /usr/share/lightdm/lightdm.conf.d/50-ubuntu.conf

Add the following content to the file

autologin-user=root
greeter-session=lightdm-gtk-greeter

sudo gedit /root/.profile

Add tty -s && at the beginning of the last line of the original file, like this

# ~ /.profile: executed by Bourne-compatible login shells.if["$BASH"]; then
 if[-f ~/.bashrc ]; then
 .~ /.bashrc
 fi
fi
tty -s && mesg n ||true

After restarting, it is the default root user