To toggle between UFO & TFTD you need to access the option from the Mods menu.
Step 1:
(https://openxcom.org/forum/index.php?action=dlattach;topic=10012.0;attach=54557)
Step 2:
(https://openxcom.org/forum/index.php?action=dlattach;topic=10012.0;attach=54559)
Step 3:
(https://openxcom.org/forum/index.php?action=dlattach;topic=10012.0;attach=54561)