The battery size for a Fossil watch varies by model. Some models use a 371 battery, according to Tmoot. To determine the battery type, note the number on the old battery.
If the battery needs to be replaced, there's no need to take it to a professional, as the tools may be found in a standard toolbox. Tools recommended by Tmoot and Made Man for replacing a Fossil battery include a small flat-head screwdriver, a rubber mallet, a soft piece of cloth, a pin remover and a battery. Made Man notes that most Fossil watches use quartz batteries, but it is best to check the size to be sure.
Fossil recommends that their watches be sent to the company if the battery needs to be replaced. As of September, 2014, the cost of doing so is about $17, because the warranty doesn't include battery replacement.