Water-resistant watches have rubber O-rings underneath their casings called watch gaskets, which serve to seal off the small mechanical and electrical components. Manufacturers of water-resistant watches warn that once opened, their watches cannot be guaranteed to perform as they did when shipped from the factory. New watches undergo a pressurised test wherein repair professionals ensure a tight seal for every new watch gasket. You can, however, remove the case back and change the batteries yourself if you take proper care when replacing the gasket.
Position the watch into the watch repair vice with its case back toward you. Clamp the case back in place with the vise's handles. You can use a rubber mat over which to work as well.
Attach the watch case wrench into the case back. Tighten the case wrench's teeth until they fit into the slots in the case back. Alternatively, press the two teeth of needle-nose pliers into two of the watch case's slots to make a tight connection.
Turn the handle of the case wrench counterclockwise to loosen the case back. Turn the handle slowly, as some watch mechanisms are spring loaded. If using needle-nose pliers, press down slightly on the pliers when turning counterclockwise to maintain a tight connection that doesn't slip and damage the case's back slots or finish.
Remove the case back and the rubber O-ring around the watch's inside lip.
Free the battery from its locking system. Your watch may have more than one battery. If the battery is held in place by removable clips, use the plastic tweezers to pull the clip out over the battery. After the clip has been removed, lift out the battery with the tweezers. For spring-loaded clips, gently lift up one corner of the clip to slide out the battery. Screw clips require optical Phillips head screwdrivers. Turn the screws counterclockwise to loosen them from the battery. Use the plastic tweezers to remove the battery. With any system, record which battery side faces up.
Replace the battery exactly as it was using the plastic tweezers. Secure their clips in place using the opposite technique you used to remove them.
Install the watch gasket. The O-ring rubber can become dry and brittle over time. If your gasket is cracked or dry, rub silicone lubricant into its surface or replace it. Insert it around the inside lip.
Close up the watch. Place the case back over the gasket and gently turn it clockwise until you feel it catch onto the screws. Insert the teeth of the case wrench or pliers into the case's slots. Turn the tool clockwise until tight.
Wear cotton gloves to minimise damage to the watch's delicate mechanism.
Metal tweezers can short out the components of the watch.