How do I set the default desktop environment on 18.04 (Bionic)?

You could reconfigure your default display manager using these steps in the terminal. I am assuming that your default is gdm3 and you want to move to lightdm

$ sudo dpkg-reconfigure gdm3
<OK>
lightdm

$ sudo update-alternatives --config x-session-manager

(you can also add other DM via

sudo update-alternatives --install /usr/bin/x-session-manager x-session-manager /usr/bin/gnome-session-classic 60 

[assuming the new session manager is /usr/bin/gnome-session-classic :-]