Pry between the bottom and top portions of the watch with a spudger in the area marked. Be sure not to insert the spudger more than about 3 mm so as not to damage the internals of the watch.

Also, there is a seal inside the watch that goes along the perimeter inside the bottom of the casing. Be sure not to damage this, otherwise you lose water resistance.

Make sure that you do not insert the spudger in any other location than the one that is marked in green.