I just replaced mine a couple weeks ago. Yours looks a little different than mine did, but it's not hard. On mine, there was a small opening next to the battery, similar to yours. On your picture, it's the top of the picture right next to the battery. Inside that small opening is a small latch where the peice of metal that holds the battery in place latches. Just use a small screw driver (like the one you used to take the back off the watch) and pop the tab off. The metal peice will pop off easily after that.
As alienator said, make sure you get it back in correctly. I'd recommend buying the battery so you can do it as soon as you take it apart. I took a battery out of a watch recently, but then didn't buy a replacement for almost a week. Took me a while to figure out how to get it back together after that.