We're a little bit late, they were replaced in February 2014, but I figured while I thought of it I'd replace the batteries in our smoke detectors. I bought two decent alkaline 9V batteries from Bunnings, and came home to put them in.
The rear one was easy, hinge it down, pop the battery out, pop the new one in. The front one though? I couldn't figure it out, so I ended up removing it from the ceiling to inspect it. It doesn't look like it even has one?
Check the manufacturer's website, and sure enough they sell one with a non-serviceable lithium battery. That makes things easier. Now why's it beeping every 40 seconds like it's got a flat battery then?
It turns out upon reading the manual for it, I must have pushed the "hush" button, and it'll beep like the battery's flat while "hush" mode is enabled. A quick test button push (and the 3 seconds of ear-splitting noise accompanying it) and it shut the fuck up for us.