If a Flood Sensor reports a low battery, we recommend replacing it as soon as possible (although you will typically have a few weeks before completely drained). The Freeze Sensor takes 1 3.5 VDC 1/2 Lithium Battery and the typical battery life for the sensor is 3-5 years.
To open the sensor, press in on the plastic above the protrusion on one of the two short ends to open the cover and reveal the battery.
Replace the 3.5 VDC 1/2 Lithium Battery from the inside compartment.
Replace the cover back on the face of the sensor.