A sensor may need to be calibrated to show the correct temperature, for instance if it is placed in a corner of the room with a higher mean temperature the the room average.
To do this, select Devices from the menu, then select the sensor you want to calibrate from the list. Select Calibrate from the menu, and insert the correct temperature. A new correct temperature baseline will be sent to the sensor the next time it wakes up. This may take upto 30 minutes.