Changing the battery or replacing the gasket on a watch requires the removal of the watch back or case. Manufacturers use different kinds of closures for watch backs, each of which requires a specific tool to remove. Removing a watch back with an improvised or mismatched tool can result in scratches on the case, marring the closure or damage to the delicate movement inside the watch.
Case Wrench
Waterproof watches often have screw-off, or screw-down case backs. identifiable by four to six notches around the outside circumference of the watch. The appropriate tool to use for removing a screw-off style back is a case opener or case wrench. A good case wrench will fit all sizes of screw-off style backs and will open the watch without causing damage to the case or internal parts.
Watchmaker's Knife
Pressure fit, also called snap-on closures, are distinguishable by a small dent or notch located between the lugs on one side of the back of the watch. The right tool to use in this case is a watchmaker's case knife, coupled with a chamois to protect your hand. In some cases, a snap-on case cannot be closed manually and requires a case press tool to reseal it.
Watchmaker's Screwdriver
Another popular type of watch closure, used with metal, rubber and plastic watches, is a four-screw-style backing. This closure is the easiest to remove, and requires a watchmaker's screwdriver, usually sold in sets to fit all sizes of watch screws. To replace the screws when you are finished, you will also need a set of tweezers to pick up the screws and set them in the holes before tightening.