As I recall, but I don't mind being corrected,
The warning and critical thresholds for hardware sensors are set on the device that has the hardware sensor. It isn't inside NPM.
NPM is just reporting what the node is indicating the current state of the sensor is.
You would need to go into the switch and see if the thresholds can be changed.
Look for something like temp > 50F = warning and temp > 120F = critical
I know from experience that servers typically have a hard-coded shutdown temperature that you cannot override. This is to protect the hardware itself.
Chris.