I can't remember how to do it either so I googled it -- I found these instructions on trip advisor:
Override or bypass for 24 hours by pressing the power and blue arrow button together at the same time. Thermostat should read Bp if it worked.
Also copied this from another DISBoard post from quite a few years ago:
On the thermostat, push the power button and the down arrow (button that adjusts the temp) at the exact same time. If you do this correctly, you will see the letters "bp" where the temp normally shows. You are now bypassing the sensors in the room. You can then set the temp to whatever you want and it keeps the room that cold whether you are in it or not. You might have to do this once a day as the thermostat tends to go back to sensor mode after 24 hours.