Getting the back off of a Fossil watch is simple. You just need the right tools. And no, you don't need to buy a special jeweler's tool or anything like that. You can remove the back of a Fossil watch using common tools you probably already have at home.
Turn your watch over and look for two notches on the back of the watch. These are for gripping the back for removal.
Using needle nose pliers, gently grip the back of the the Fossil watch in the notched areas.
Turn the back clockwise, keeping the pliers in the notches.
If you can't get the back off your Fossil watch with pliers, try a flat head screwdriver instead. Slide the flat head of the screwdriver into one of the notches and push it clockwise.