How do i open the back of the watch to change the battey?

There are typically 2 ways to open the back of a watch. Some watches have a snap off cover, other are screwed on.
Snap On Somewhere around the perimeter of the back of the watch you will see a small crescent. This is the place where a small screwdriver can be inserted and twisted to remove the back.
To replace the back, simply position it evenly on the back of the watch and press.
Screw On Removing a screw on back is slightly more difficult. For these backs, there are typically a series of slots around the perimeter of the back of the watch. The best way to loosen the back is with a pair of needle nose pliers. Place the 2 tips of the pliers in slots on opposite sides of the back of the watch. Apply light but constant pressure and turn the back counter clockwise.
To replace the back, carefully replace the back with your fingers. Be sure the back is set correctly in the threads by turning the back slightly counter clockwise. When you feel the threads click into place, simply turn the back clockwise with your fingers to tighten. Once finger tight, once again use the needle nose pliers to tighten the back.