If you've discovered that your watch is losing time or has stopped completely, then you're probably in need of a new battery. In order to replace the battery, or to further examine the watch to ensure it's working correctly, you'll have to remove the back case. Usually, this is a simple task, as most watches have a snap-on back. Other watches, such as those with a retainer ring and screw-on back, require more work and special wrenches. If you own one of these watches, it's best to take it to a professional for servicing.
Position your watch so that it is face down on a soft cloth. This is so you do not scratch the watch face.
Examine the back of the watch and find the small groove that runs between the body of the watch and the back case.
Slip the edge of your knife gently into the groove. Most often, you will have the best success by inserting the blade near the 9:00 position. Be careful not to push the knife too far, as it may damage the interior workings of the watch or the waterproof seal.
Twist the knife blade slightly, lifting the back case upward. The case should pop off with very little effort.
Position the watch so that it is face down on a soft cloth.
Insert a small coin (penny or dime) into one of the slots on the back case of the watch.
Turn the coin counter-clockwise until you've loosened the screw. After the screw has been loosened, remove it the rest of the way with your fingers.
Repeat the process on the other hatch screw. After you have both screws removed, you should be able to lift the back case off of the watch. If the case does not lift off easily, use a knife with a slender blade to gently work it free.
If you own a watch with a screw-on back case or with a retainer ring, and you have the proper tools (rubber-tipped opener and retainer wrench), you can open the watch backs on your own. For a screw on back, firmly press the rubber-tipped opener in the middle of the case and turn it counter-clockwise until it unscrews completely. For a retainer-ring watch, align the notches on your retainer wrench with the slots on the back of the watch. After the wrench is in place, turn it counter-clockwise until it is completely removed.