change default auto login user in ubuntu

sudo leafpad /etc/lxdm/default.conf
sudo leafpad /etc/lightdm/fileexample.conf

In the fist few line, you will see something like autologin=username change it to the original state of # autologin=dgod. Save the file, and then reboot your machine.

autologin-user=username
autologin-user-timeout=0

Using privileged admin account open up terminal or your favorite text file editor and edit the /etc/gdm3/custom.conf configuration file. Change from:

[daemon]
# Uncoment the line below to force the login screen to use Xorg
#WaylandEnable=false

# Enabling automatic login
#  AutomaticLoginEnable = true
#  AutomaticLogin = user1