First we look at what we are currently running
$ gsettings get org.gnome.desktop.session session-name
That will echo us ‘gnome’ if we running Gnome Shell or ‘gnome-fallback’ if we are running fallback.
To change it (within a Gnome Session) from Gnome Shell to Gnome Fallback
$ gsettings set org.gnome.desktop.session session-name 'gnome-fallback'
Of course this is not useful at all, because if we can inside a Gnome Session we can simply change it by
System Settings -> Details -> Graphics -> Force Fallback Mode "On"
But if we cannot start a Gnome Session we can do it from console (TTY) by using dbus-launch
$ dbus-launch gsettings set org.gnome.desktop.session session-name 'gnome-fallback'
Useful and easy!