To replace batteries in a Casio watch, open the back case of the watch, remove the old battery, insert the new battery, and then close the case. For this to work, you need a jeweler’s screwdriver set and a pair of tweezers.
Casio watches have three types of back plates: the screw-down, snap-off and screw-off. To open snap-off cases, place a flat screwdriver along the edges of the case, and lift it up to open the case. For screw-off cases, hold the watch, and turn the back case to open it. Use a screwdriver to open the back case if it is fastened by screws.
To remove the battery, gently open the clasp holding the battery. If the battery is held by a latch, use a screwdriver to loosen the screws. To avoid losing the small screws attached to the latch holding the battery, do not unscrew them completely. Use a pair of tweezers to remove the old battery.
Insert the new battery, and make sure the correct side faces up. Fasten the screws holding the latch to make the battery firm in its position. Slowly turn the watch over to check whether it is working. Place the Casio watch case back to its position, and fasten it.