Can't un-pair a bluetooth device

Hey, I have a Magene sensor, it works for cadence and for speed, we just have to select in what mode we want to use it. By error, I connected the sensor in cadence mode, but I want it in speed mode, now I unpair the device but I still can not connect the device in speed mode. Can somebody help me to reset the recognized devices or do something to forget the device and start again.

The leaflet says: Remove and reinstall the battery. The mode will change then automaticly.
Watch inmediatly therafter for the led: red = cadence mode; green = speed mode. succes!