The following guide is to show you how to change your default display manager on Ubuntu 18.04/18.10. This could be because you wish to switch back over to Unity, which was removed from Ubuntu 17.10 and up.
1 Check the default display manager
To begin, you need to check the current default display manager.
Open the GNOME Launcher, click on Terminal and type in the command below:
Ubuntu 18.04.1/18.10 should return:
2 Install LightDM (Unity) on Ubuntu
Applies to Ubuntu 18.04/18.10:
To install LightDM, type the following:
sudo apt install ubuntu-unity-desktop
Input your password at the authentication prompt. The package will download and install.
3 Configure LightDM
You will then arrive at the Configuring LightDM screen
4 Select default display manager
Select LightDM to setup Unity as your default (use the arrow keys to select LightDM then press tab to get to the OK button and press enter)
5 Check which is the default display manager
After the configuration is complete, you can double-check your default display manager is now set to LightDM.
This should return the following:
You can now log off/reboot to switch over to Unity.
6 Changing back to GDM
If at any point you would like to change from LightDM back to GDM, open the GNOME Launcher, click on Terminal and type in the command below:
sudo dpkg-reconfigure gdm3