The battery of my Casio 1846 W-71 was dead, so I tried to replace the battery by myself.
First, you need to get a hand on a CR2016 3-volts battery. It could cost you 5$ or 10$, but I got 5 of them for 1.50$ at an electronics discount store...
Get a precision screwdriver, remove the 4 screws. Be careful when you remove the cover, there's a tiny spring. Remember where it is, then remove it. Remove the watch from the shell.
Using a flat mini screwdriver, remove the cover that holds the battery. You'll need to unclip all 4 corners. There are centers to unclip too, not sure if there was 2 or 4 of them.
Remove the battery, place the new one. Clip the battery cover back, and put the watch back in the shell.
I closed the cover. The watch was showing time, buttons were working, but no sound at all. Looked on the internet, and read that you need to short circuit it.
Look carefully, there is a label "AC" near a small hole. (Inside the hole you'll see the gray spike of a welding). Just touch something metal inside the hole (e.g. the welding spike) and then touch the positive (top) side of the battery. Put the cover back in place, press a button and listen for the beep.
Screw the cover back in place.
It seems that your watch is not waterproof anymore after you open the cover. It would certainly not resist 50 meters underwater anymore, but I wear mine in the swimming pool without problems.
Yves