It's a 395 (SR 927 SW), however you're going to need a case wrench and a 4-pin vise to get the back off so you're looking at a minimum of $40 for the battery and the tools and that's if you know where to look. The other problem is the rubber gasket which may need to be replaced as well. Unless you happen to have a supply of them handy for that exact size case, your watch will no longer be water resistant to the manufacturers specifications. You could probably wash your hands with it but I wouldn't recommend anything more intense. The problem that you have is that you won't know if you need a new gasket until you get the case open and as noted, that's cost you a minimum of $40 to find out and you get the pleasure of having a set of tools that you'll probably never use again.
Or, you could save yourself the money, headache and aggravation and of course the high probability of you damaging it because you lack the experience to change the battery on a screw-back, water resistant timepiece and just take it to a local jeweler, pay $10 - $15 to professionals who know what they are doing and will include the replacing of the gasket at no extra charge.
A universal Screw-Back (what your watch has) Watch and a Case Wrench
A 4-Pin Vise