The WEATHERmeter uses a replaceable CR2450 battery.
The battery lasts for about 500 hours of use on low power, and about 300 hours on high power, which should be many years of normal sampling.
If the battery needs to be replaced, peel off the silicone sleeve from the top part of the meter to access the battery. Slide the battery door off, replace the battery, and slip the sleeve back on.
If the battery is removed the compass will need to be re-calibrated!