Switch to root user

sudo passwd root 

The root user of give new password.

su root

(Switch User) Change active user.

Switch to root at interface

nano /etc/lightdm/lightdm.conf

And you find #greeter-show-manual-login=false later just remove # letter and set false to true (finnaly you should press CTRL + S).

To use Desktop

Temp Method

caja --force-desktop

Permanent Method

Step 1 - Open startup applications.

Step 2 - Add startup program. (like this)

Delete user

userdel -r -f username

-r : remove

-f : force