1. Remove the sensor cover. ( See image below )
All sensors clip off except the motion sensor that slides down.
2. Replace the battery ( Battery types listed below).
For all sensors but water sensor the battery slides or clips out.
To remove the battery on the water sensor, it requires a small Phillips head screwdriver.
Replace the battery (the positive side facing you).
3. Replace the sensor cover.
Motion sensor: (2) AA Batteries
Water leak sensor: CR2450
All other sensors: CR2032