The Seiko 7N42-6139 is a "snap back style watch. "Snap back" refers to the type of mechanism used to hold the watch back to the watch face. These types of watches are simply fitted together with a pressure seal. With the right set of tools, removing the back of the Seiko 7N42-6139 is a straightforward process. Once the back is removed, replacing the battery takes less than a minute.
Grip the watch face down in one hand with the piece of leather. The leather helps to hold the watch firmly without damaging the face and protects your hand from the knife or screwdriver.
Insert the bench knife or small screwdriver into the recess between the watch back and the watch face and begin to pry the two pieces apart. Rotate the watch as you pry until the back pops off.
Set the watch face down on the piece of leather. Remove the battery with the plastic tweezers, taking care to note which direction the battery is facing.
Insert the new battery, in the same direction, with the plastic tweezers.
Line up the groove in the watch back with the stem of the time adjustment knob. Grip the two pieces in the leather and push the watch back firmly into the watch face. Hold the leather in such a way as to exert maximum force on the watch back without directly pressing on the watch face to avoid cracking and other damage.