Here is a video on changing the battery pack. https://youtu.be/heckZRihcRw
Remove the sensor stick from the bottle lid. Unscrew the lid from the the sensor stick. Remove the old battery pack and dispose of it appropriately. Insert the new battery pack. Make sure that you insert the battery pack the correct way within the lid. The side that has two metal contacts is the bottom side that will come in contact with the metal rings on the stick portion. Then screw down the cap keeping the battery pack in place. Make sure to screw it down as tight as possible to ensure a water tight seal. The sensor stick will glow if you have inserted the battery pack the correct way. If it doesn’t glow then try inserting the battery pack the other way. Re-attach the sensor stick to the lid and you’re good to go