The battery number will be imprinted on the dead battery (for that watch it's most likely 379), however, I wouldn't recommend replacing a battery on a Fossil yourself. They are notorious for their back being incredibly difficult to remove and put back on and if you're not a professional, you will most likely break the crystal in the process. If it's a screw-back (and you can tell because it will have six rectangular shaped notches around the edge of the back) you won't be able to get it off unless you have a case wrench and a case vise. Take it to a jeweler and have them change it for you.
Screw-back Style Case