Open a terminal.
sudo nano /etc/lightdm/lightdm.conf
It displays some text as follows:
[SeatDefaults]
greeter-session=lightdm-gtk-greeter
user-session=mate
autologin-user=username
Solution
This would be your particular user name that is automatically logged in with or without password. Delete this username and type in the administrative username or leave it blank.
Shut down the system. It will go back to the login selection screen. Just power down the system and put it on again.
