at the back of the watch face you should be able to pop the cover with a small screwdriver or sometimes its one that you have to screw off - and you should be able to see the battery there - take it off and take it with you to to the watchmaker to get another new battery or just take the serial code number down and replace it.
howtomendit.com for free repair help, information and advice.
Try fixfault.com to fix wrist watches.