Changing the default window manager in Linux.

How to change the default window manager (KDE, Gnome, fluxbox, etc.) in Debian:

Symlinks in /etc/alternatives point to various "default" programs.

To change the default window manager look /etc/alternatives/x-session-manager symlink.

You can also use
update-alternatives --list x-session-manager
update-alternatives --config x-session-manager