Disarm your Xfinity Home security system and put it in the test mode before you start to replace the batteries. Then remove the cover from the sensor to expose the larger compartment with the battery in it. Make a note of the positive and negative on the batteries already in the sensor. Replace the old ones, and place the cover back on the sensor. You can ignore the home system alert which tells you the sensor is being tampered with. The alert will automatically clear out when you are done replacing the batteries.