Casio watches are electronic and electronic-mechanical. They are significantly less sensitive to dust than mechanical watches with spring drives. Therefore, you can open them at home and change the battery yourself.
Despite the slight sensitivity of Casio watches to the ingress of fine dust, prepare the workplace so that there is no large dust visible to the naked eye on it. Provide good lighting. Please note that after you replace the battery yourself, the watch will lose its water resistance. If it is important for you, use the services of a workshop. Do the same if the watch is under warranty.
The bottom covers are attached to the watch in different ways. If it is just tightly inserted into the case, pry it off with a thin slotted screwdriver and carefully pull it out. Unscrew the screwed cover. If it is supported by four screws, carefully remove them with a miniature Phillips screwdriver. When separating the cover, do not lose or break the rubber seal. Under no circumstances should the watch be clamped in a vice, both during disassembly and assembly.
Pay attention to how the battery is installed. Usually it is facing upward. The latter is connected to the body of the element. Gently pry it with a dielectric object and pull it out. Move the upper flat spring forward if necessary.
Place an open watch in a clean, sealed box. For a new element, go to the underground passage - most often they are sold there. Do not try to purchase a battery from a watchmaker - they will charge you with the price of the battery and the cost of replacing it, although the latter was not produced. Show the item to the seller and buy the same. Don't short-circuit it.
Insert the battery into the watch, observing the polarity, and, if necessary, press down on top with a flat spring. Make sure the device is working. Reinstall the cover in reverse order, not forgetting the rubber sealing gasket. If the lid is pressed into the case, be careful not to crush the glass on the opposite side of the watch. Check the operation of all controls and set the current time.