The battery level of your sensor can be checked on the My Valuable's page using the battery icon located in the upper right hand corner next to the Bluetooth icon for each sensor. Notifications will also be sent once your battery level has dropped into the critical range.

The sensor battery can be replaced by simply removing the battery door on the back of the sensor, sliding out the old battery and sliding in a new CR2032 battery. When installing the new battery, make sure the flat + side is facing up.