I found the intruction on how to calibrate the Jager heater

1) Put your heater in sump water of a known temperature for 30 minutes.
2) Turn the heater dial clockwise until the lamp turns on.
3) SLOWLY turn the dial counter clockwise until the lamp just turns off.
4) Using a flathead screw driver pry the calibration button on the top, as clearly shown in the picture above, until the button pops up.
5) Turn the heater dial so it points to the known temperature of the water the heater was in and forcefully push the calibration button back in.