Is it a screw back or a snap back? Screw back should have at least two small notches on the back. You can use a small flathead, spudger, or a case wrench to open it. Insert it in the slots at the same time and turn counter clockwise. Snap back can be opened with any of the same tools inserted in the notch between the watch and the back case. Give it a little lift and it should pop right off. This link goes into more detail about the battery replacement. http://www.ehow.com/how_6466795_replace-watch-battery-dakota.html
Hope this helps! ~GG