Two years after I purchased a Casio Duro MDV-106 watch (sometimes call "The Marlin") on a clearance deal at a local Walmart store, it was finally time to change the battery. I had never opened up this watch before so, if you like, please join me as, for the first time, I see what's inside this case. The watch uses a Mercury-free SR626SW battery (also sold as a 377 and under other designations too). This whole task was accomplished with minimal tools and only basic watch repair/maintenance skills. I highly recommend you acquire an official case back tool for unscrewing the back cover of any watch with a screw-down case back. They're fairly easy to find online and you can often find a kit with several tools, including the proper tool for unscrewing a case back. They don't have to be expensive to do the job correctly.
As I explain in the video, if you're not confident about doing a battery change yourself then I recommend you go to an experienced jeweler or even someone who works at a dedicated watch/clock store at a shopping mall. They'll probably charge you the same price you'd pay at a jewelry counter at a local Walmart store but I suspect that the non-Walmart service will be more reliable, as you' be dealing with someone who has experience with watch maintenance on a daily basis.