Raspbian Stretch choose default desktop to be XFCE

Execute alternative dialog:

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

This will show options. I will choose 6 for xfce4:

There are 6 choices for the alternative x-session-manager (providing /usr/bin/x-session-manager).

  Selection    Path                      Priority   Status
------------------------------------------------------------
* 0            /usr/bin/startlxde-pi      90        auto mode
  1            /usr/bin/lxsession         49        manual mode
  2            /usr/bin/openbox-session   40        manual mode
  3            /usr/bin/startlxde         50        manual mode
  4            /usr/bin/startlxde-pi      90        manual mode
  5            /usr/bin/startxfce4        50        manual mode
  6            /usr/bin/xfce4-session     40        manual mode

Press <enter> to keep the current choice[*], or type selection number: 6