These smoke alarms were already placed in the house I am renting, I need to change the battery in one of them but cannot figure out how to get to the battery. I took it off the wall and unplugged it, the top won't twist off or lift off, I just don't want to break it by prying the top off. Will you please tell me how to change the battery in this thing, I have attached a picture. All it shows is model H.