To remove and replace sensor stick, tip bottle upside down and remove bottom plastic ring. Then you are able to remove the entire sensor stick.
First, check sensor stick's black gasket making sure that the black seal/gasket is sitting on the base of the sensor with the FLAT side down and the upper side (facing up towards sensor stick) is tucked neatly into the groove/ridge around edge of the white circular sensor base. Make sure your cap/lid is on bottle and turn bottle upside down. Now, place sensor stick into bottle keeping rounded side of the sensor stick facing the lid's mouth piece for optimum glow visibility. Place bottom ring on tightly, and you are good to go:)