Put the thermostat in Off mode
While holding down the reset button, press the down arrow button until the offset indicator appears and release reset. Press the reset button again. (non-programmable)
While holding down the reset button, press the set key until the offset indicator appears. Press the set button twice. (programmable)
The unit should say HP (if it says F, press an arrow key to toggle to HP)
Switch mode to heat or cool.