As I recall, first you need to be in Calibrate Mode (holding down the first two buttons together for three seconds?); then you toggle through all the different Calibration windows until you get to Water Temp; then you press the third (or fourth?) button once to switch C to F or visa versa... I have the same unit and I'm very pleased with it, especially for the price. As for the manual, well, not so much. I think the whole world should pay for the LEGO company to write their manuals!