The Tilt Sensor uses a single CR2032 battery that typically lasts 3-5 years.
To open the sensor, insert a tool in the indentation along long side and pry to remove the cover from transmitter.
Pop the CR2032 battery out of the compartment and replace with positive end facing up.
Re-place the cover back on the sensor. It is important the cover goes back on the same way it came off. The way you can tell is, on the inside of the cover there is a small plastic piece that you want to align with a black button on the inside of the sensor.