Here is a video on how to properly remove and reattach the sensor stick.
Removing the sensor stick
Unscrew the cap of the bottle.
Grab the stick close to the lid firmly in one hand while holding the bottle cap in the other hand.
Now in snapping motion remove the sensor from the bottle lid at an angle. It is a snug fit so you will have to use force and may feel like you are breaking it, but don’t worry it is pretty durable. Also don’t try to pull it straight out as that may damage the seal on the sensor stick.
Re attaching the sensor stick
Before reattaching the sensor stick, make sure the sensor stick cap is SCREWED DOWN AS TIGHTLY AS POSSIBLE to ensure a water tight seal. There should be almost no gap between the sensor stick lid and the stick. Remember only unscrew the sensor lid when replacing the batteries.
Now take the bottle lid and make sure it is clicked closed and place it upside down on a flat surface.
Make sure the rounded side of the sensor stick is facing the mouth hole so you can see the light when it glows.
While holding the bottle lid firmly in place align the sensor stick at an angle and push down while positioning the stick upright.
Here's a quick gif to show you how it's done.