W to changebattery #65041g

Changing the battery on most watches is fairly straightforward and needs only the usual tiny screwdriver, insulated tweezers and a large measure of patience and good eyesight.
The main problem is usually opening the case and in the case of a quality watch that is guaranteed waterproof it really should be returned to the manufacturer for battery fitting and pressure testing. If testing isn't carried out after the case has been opened the watch realistically then becomes merely water resistant rather than waterproof.

Usually it is the back of the watch case that must be opened and in the case of ordinary non-waterproof and non-water resistant types it is a simple matter to prise the back off with a thin blade and then refit it with finger pressure.
Water-resistant watch cases that have a priseable back often need the back pressing back on with force supplied either by a purpose-made press or a large bench vice. In both cases adapters are needed to prevent damage to the case and the crystal.

A lot of watches have a back that unscrews and strictly a dedicated key, wrench or spanner is needed for this task. I have managed to overcome this problem a number of times by pressing other tools "that look as if they might do the job" into service. I cannot say what it was I used as I wandered around my workshop just trying things. I think the last time a particular circlip plier did the trick.

Simpler to take the watch and have someone else replace the battery I think.