I recently wanted to change the EL button on one of my G7900s, but on searching, I couldn't find a thread although I could swear I've seen something in the past. Anyway, went ahead and did it. My initial advice is don't! This is the trickiest most irritating mod I have done. I remember some previous advice which said to do this inside a ziplock bag - I tried and found it impossible. The optical quality of the bag is so poor that I couldn't even focus on the circlip properly! Admittedly, if you've much better eyesight than me, this may not be such an issue.
This is the circlip - also called the 'e' clip by Casio...
To gain access to the button, you need to remove the module from the watch. This is the easy bit, simply remove the 4 x screws at the side holding on the bezel, and ease the bezel over the side buttons, it's quite flexible and you won't break it.
Then, remove the watch back - keep these screws separate as they are different to the side ones. Gently remove the plastic module guard, and take note of the way it is orientated. It needs to be located with the hole directly over the 'alarm' spring, otherwise you'll insulate it from the watchback and lose your alarm.
Take out the guard before getting access to the module...
Once, you take the guard out, then ease the module out, taking care not to scratch it, lay it down on a soft cloth, out of the way of your immediate work station.
You will then gain access to the 'e' clip (circlip) that holds the light button on. This is the tricky bit. The 'ziplock' advice is handy here as the clip will spring off quite sharply. It just needs eased out with a very small screwdriver. The old button can then be pushed out, but take care as there is a small spring located at the base of the button stem.
The holy grail - 'e' clip in place...
The trickiest bit is installing the new button. I found it easiest to insert the new button, making sure that the top indent on the stem was flush with the internal case. Then, whilst resting the clip on the case, slide the 'e' clip onto the stem -this is the bit that will have you cursing. Be careful not to apply so much force that the screwdriver slips and can potentially scratch the inside of the glass (no, of course I didn't do that! But, I was dam close )
Button with spring in separate bag
When the clips eventually locates itself, you will instantly get a warm glow all over and swear that you'll never open another watch!!