Open Terminal and run:

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

There are 3 choices for the alternative x-session-manager (providing /usr/bin/x-session-manager).

Selection Path Priority Status
————————————————————
0 /usr/bin/startxfce4 50 auto mode
1 /usr/bin/mate-session 50 manual mode
* 2 /usr/bin/startxfce4 50 manual mode
3 /usr/bin/xfce4-session 40 manual mode

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