The best and safest way to change a G-Shock battery is to bring your watch to an authorized Casio dealer/jeweler. He has the right tools to open the watch, new O-rings, in case the watch is not water-resist anymore and he can perform a 200 m water resist test. If a G-Shock is damaged due to these kind of operations within 1 or 2 year after purchasing the watch, the warranty will not longer be valid! It's most of the times easy to see if a G-Shock is opened by a non-professional watchmaker!
For people living in a place, far from a Casio Dealer it is possible to change a battery with the proper tools, but there are a lot of things that can go wrong! Most of all, you can damage the O-ring. In that case your G is not water-resistant any more. I can tell you, they die, and won't revive if there comes any water in a G-Shock module.
Note: The photo's shown here are specific to the DW-9000. Other models may have slight differences.
How to change the Battery:
If you remove the back plate, you will see the back shock absorber. Carefully remove that rubber pad, and remind how to put it back. There is also a tiny spring. It is put loose in a little hole, so, NEVER turn your watch/module up side down! If you lose that spring, the alarm (s) won't work anymore.
Opened back of an DW-004. You can see the black shock absorber. The tiny spring is clear to see. Note that this spring can be located on other places on different modules. Some modules do not have a spring. At the bottom you'll see the EL contact lip.
Under that shock absorbing pad, you'll find the battery locked in the battery holder.
Put an unfold staple or a very small watchmakers screwdriver in the tiny space between the two white lips (left). If you do that right, the battery lock will open (right).
Most of the times it's a CR2016. You'll need a very small watchmakers screwdriver, or like I did an unfold staple to unlock the lock. Don't force it, or it will deform, damage or break of! If you push the right pin, the lock jumps open, and you can carefully take out the battery. Remind, don't turn the watch upside down.
Put in a fresh battery and close the battery lock.
Check if the watch is working properly (remind still don't turn the watch)..
If the display shows a normal time you can close the watch in the opposite order as you opened it.
If the watch displays a strange looking display, or symbols that not look normal, you've got to reset your watch. I use a watchmakers screwdriver and a fold open staple. Find the contact marked with AC.
The AC contact
Carefully put the fold open staple in the AC contact hole. Use the screwdriver to short circuit the staple with the back of the battery. If you look at the display, you will see that the watch shows it initial time (something like 12.00.00, date 1-1-'95). Now you can close your watch in opposite order like you opened it.
Resetting the module by connecting the AC contact with the back of the battery.
Take care of the rubber O-ring (the best way is not touching it), and put the back plate in the right position (not upside down. If you have put the back plate in place with the four screws, check the alarm and the EL light. If the alarm doesn't work any more, it's possible that you have lost the spring. If the EL doesn't work, it means you have bend a contact on the module, or must have damaged the EL. The first error is to correct by carefully bend the little metal contact outward, and clean the place where the contact hits the back plate.
the EL lip. This must make a good contact with the back plate.
Altogether it's a bit risky operation. Never try to replace a battery or open a back plate on a:
Tough Solar G-Shock's,
Analog or analog/digital G-Shock's
G-Shock's with an EPROM memory, like certain G-Cool's and Codename's
G-Shock's with sensors, like a thermometer, altimeter, compass and/or barometer. You'll probably damage the watch already when opening it!
I have changed some batteries of very old G-Shock's that I don't wear, and practiced it on a damaged G-Shock, that I use for spare parts. Still I recommend to bring it to your local Casio Dealer!!!!