We are renters. Our thermostat has no kind of modal number of any kind showing. There is one small square screen that shows the current temp. It also has 2 buttons with up and down arrows. THAT. IS. IT!!!! Can't pop off any cover for further examination. Nothing!! Please help!! It's getting cold where we live and we can't figure out how to switch to heat. Have tried holding both arrows down, it gives us numbers that go 1 thru 5. But no heat or any other change. HELP!!!!!!