The battery level of your sensor can be checked on the My Instruments page using the battery icon located in the sensor summary next to the temperature and humidity icons. 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.
* Make sure the sensor blinks 4 times after changing the battery.