The display manager’s role is to provide graphical login functionality. If you have both GNOME and KDE installed on your system, you can easily switch between the two display managers to find one that meets your needs.
1, Open a terminal window.
2. Execute the following terminal command:
sudo dpkg-reconfigure gdm
3. In the Configuring gdm section, hit RETURN on the keyboard to select the OK box.
4. You will now select the default display manager, select either gdm or kdm, then select OK for your changes to be accepted.