MY machine is running Debian 8 Jessie.I installed Gnome desktop environment bysudo apt-get install gnomenow every time my machine is switched on I'm presented with Gnome desktop environment whereas I want to go back to KDE (login screen doesn't let me change).How to fix this?I tried with sudo  update-alternatives --config x-window-manager but apparently the windows manager is just a part of the whole GUI system.
Debian: How to revert back to KDE after switching to GNOME?
debian;gnome;kde;window manager
I doubt that your display manager doesn't let you choose, but I don't like neither Gnome nor KDE, so I don't know what you'll be running, but you can try some others. Personally I like lightdm, but there's a list at https://wiki.debian.org/DisplayManagerA less subtle approach would be to uninstall gnome.