The HM-1 take a Panasonic CR2477 3V lithium coin cell battery. This is a high-capacity battery and is rather widely available. You may google "CR2477" to find this and equivalent batteries.
The battery door is located in the back of the sensor. Turn the door counterclockwise to unlock the door. Then, to assist the removal of the door, first knock the sensor on a hard surface. This will loosen the door and make it easy to remove it (not knocking it makes the door removal a challenge).
Remove the battery and replace it with the + side up ( - side down in the pocket).
Attached to this article is the CR2477 datasheet.