Casio electronic watches will run for about a year before the battery must be replaced. You can change out the button battery yourself with a jeweller's screwdriver and a steady hand. Replacement batteries are available at watch and jewellery shops and electronics stores.
Fold a cloth or towel and place the Casio watch face down on this padding to prevent scratches and small parts from rolling off the workspace.
Remove the four screws securing the metal plate to the back of the watch. Set aside the screws in a safe location, such as in a plastic cup.
Lift off the back plate of the watch cover and the rubber water seal gasket beneath it. Set these parts aside in a safe spot.
Locate the battery, which is slightly off-centre inside the watch cover to the left. There is a plastic latch across the top of the battery, securing it in the compartment. Use the screwdriver to pry up the latch from the right side of the watch compartment and swing it upward and to the left. The battery will pop right out. Note that the battery is installed positive (+) side down.
Take the old battery with you to the store to match up a replacement.
Insert the new battery positive (+) side down in the compartment and swing the latch over it. Press gently on the latch so that it clicks into place.
Align the rubber water seal gasket with the watch case and place the back cover over the gasket.
Secure the cover to the back of the watch by tightening the four screws.
Set the time on your watch and you're ready to go.
Leave the Casio facedown on the cloth or towel at all times until the battery replacement is complete.