Go on-line to that "A" store & look for a watch battery replacement kit. There are kits as low as $7.00.
There is a tool that slips under the case & pops it off. Most watches use that type of tool. There are some watches that require a different tool.
Batteries are available at a grocery store.
I have been changing watch batteries for years & you will save a bundle. Jewelry store charge quite a bit to change batteries.