I have had a Casio Protrek PRG-40-3VUR for about three and a half years now. It's a great watch. Recently I noticed that the compass was no longer working and was displaying an error message. A bit of Googling showed that the batteries needed replacing, hardly surprising after three years use. Replacing the batteries isn't too difficult but is quite fiddly, so take your time.
There are a number of articles on the Internet that discuss how to replace the batteries in Casio watches, but none specifically for this one. I didn't even know exactly what batteries were required, so I decided to see if I could take the watch apart to find out.
The only tools you need are a jewellers screwdriver for the Philips screws that hold the case together, a small flat bladed screwdriver, or a pair of tweezers to remove the batteries.
There are eight screws that need to be removed. Four on the back of the watch and two on each side. These are very small, so it's best to perform this operation on a piece of white paper, so you can see them if you drop them. The screws on either side of the watch are different, so make sure you note which side they came from and put them back in the same side.
Once you have removed the screws lay the watch flat on its face and gently remove the grey plastic back cover. Underneath this is a rubber gasket that forms a watertight seal. Be careful not to damage or lose this. You can now carefully lift off the metal back and the white plastic cover underneath it. You should now be able to see the four SR927W batteries that power the watch.
These can be quite tricky to remove. You will need a small flat bladed screwdriver, or the point on a pair of tweezers. The batteries are covered by a small clasp which must be opened, so you can remove the batteries. Each clasp has a small hole near the centre. If you insert the blade of your screwdriver, or the end of the tweezers into this and push it gently towards the centre of the watch the clasp will release. The clasp will only lift a small way up, don't try and force it too far. Now gently prize the batteries out.
Put in the new batteries. Don't do this using tweezers, or you will probably short them out. Make sure they are properly seated and gently re-seat the clasps.
You will now need to reset the watch. There is a small sticker with red print stating: "After battery replacement, contact AC with terminal (+) using tweezers". It's not at all obvious where the AC terminal is situated. If you look carefully there is an arrow engraved on the metal plate inside the watch pointing at it. It's a small gold plated terminal inside a hole pointed at by the arrow. There are four contacts in the hole, make sure you pick the one the arrow is pointing at. I did this using a short length of wire. Carefully holding the inside of the watch to prevent anything falling out look at the face. You should see that the watch is working.
Now replace the white plastic cover and metal back. Carefully insert the gasket in the groove in the metal back and replace the grey plastic back without disturbing the gasket. Now replace the screws, being careful not to over-tighten them. Finally reset the time and date to the correct values.