[Cross posted from Photography Hack]
This worked perfectly tonight with my Canon S400. Be careful to disassemble the bare minimum necessary. If you see a spring popping up, you are disassembling too much. The PDF is helpful but the commentary below from the DP Review Canon S400 forums is really what counts. Make sure that the sliding switch for the take/review function is correctly located before you re-assemble as noted below. I didn't and had to dissamble it again. The other problem I had was screw #13 on page 4 of the PDF. It was hard to get it in. Barb managed to do it. I struggled unsuccessfully for over half an hour. All in all, it was worth it. Now I don't have to buy a new point and shoot. I think that if I were, I'd seriously consider the brand new Panasonic Lumix DMC-TZ1 (my only concern is the high ISO, is it noisy?).
The only tool required is a small screwdriver. I would not class the procedure as difficult but you need to proceed with care on a well-illuminated, flat, plain surface. A pencil and paper are useful. Remove the rechargeable main battery and the memory card.
You then need to remove the small screws around the casing. These screws are of different lengths and it is important to lay them out in a logical manner so that later you can replace them correctly. Carefully separate the halves of the casing. This reveals the internal disk-shaped 3-volt battery. You can use as a guide the exploded view of the camera shown at
The 3-volt battery is part 3 on page 4 of the exploded view diagrams.
Remove this battery, taking care not to touch both faces simultaneously with a metal implement. After about an hour, replace the 3-volt battery.
Reassemble the metal case taking care that the sliding switch for the take/review function is correctly located. Replace the main rechargeable battery and the memory card. You will have to reset the time and date data as they are deleted when the 3-volt battery is removed.