How To Change The Default Display Manager (Switch To GDM, LightDM, SDDM, Or LXDM) In Debian, Ubuntu Or Linux Mint

When installing a display manager on a Debian-based Linux distribution, you're asked which display manager should be used by default, but it may not be clear for some users how to change the display manager later on.

GDM3 (Gnome Display Manager) in Ubuntu 18.10

A display manager, or login manager, is a program that manages graphical display servers and handles user logins. This article explains how to change the display manager, from / to GDM (GDM and GDM3), LightDM, SDDM or LXDM. The instructions below should work on any Debian and Ubuntu based Linux distribution, including Linux Mint, elementary OS, and so on.

This can be useful in case you're having issues with your current display manager and you want to use a different one, in case you installed a desktop environment that uses a different display manager by default and you want to switch to it, and so on.