Ubtuntu 16.04 change default desktop environment

First get the available desktop enviroments.

ls /usr/share/xsessions/

Let's say your output is: ubuntu.desktop and xfce.desktop

Then modify the following file:

 sudo nano /usr/share/lightdm/lightdm.conf.d/50-ubuntu.conf

To make xfce default, replace "ubuntu" with "xfce", save and reboot.