I read the item on changing the battery years ago, so had a try on my original, but decided that I was in danger of irreparably damaging the box and left it.
Perhaps one day.......
Edit - success! I couldn't leave it alone. I think that there is a U - shaped piece of card which is (maybe) glued along one side, so a suitable length of wire bent at 90 degrees was inserted at several places along one side, then the other, to see which on moved most, then that side was slowly pulled up until the cardboard was straight/flat along it's length, revealing the circuit board. This was removed. Mine was still as new, no corrosion, so a new battery inserted and bingo! The board was carefully slid back into position, taking care to align the led, pushed fully home, and the cardboard case was manipulated back into place down the side of the circuit board.
Quick check, still going. Having suitable tools definitely helped.
Hope this helps.