The FYTA Beam G2 (recognisable by its matcha green housing) uses a CR2450 coin cell battery.
CR2450 batteries are available at supermarkets, pharmacies, and online. Battery life is typically 9-12 months, depending on ambient temperature and how often the sensor connects. When the battery runs low, the sensor flashes red when you press the button - and the app will notify you too.
To replace: open the battery compartment on the back of the housing, remove the old battery, and insert the new one with the correct polarity. The connection to the app will restore automatically.