Okay, so I’m a geek and I wear a calculator watch (most of the time). I’ve been sporting the Casio CA53W-1 since about 1980. I’ve had several.
Replacing the battery is not good for over 40 eyes. You definitely need some magnification! If your remove the 4 screws from the back you have a tiny o-ring that seals around the back and a CR2016 battery that is held in place with a clip. There is also has an insulator attached to the clip.
Push down on the battery to slightly flex the clip and use a small jeweler’s screwdriver to carefully detach the clip side that is located toward the keyboard. Once you hook it, it shouldn’t take much. Then, carefully remove the clip and insulator.
Place a new battery in the compartment, + side up, and hook the clip/insulator to the top hook and then press down to flatten the clip. Use your small screwdriver to carefully assist with attaching the bottom part of the clip. Don’t rip up the insulator!
After installation, you’ll have to perform a reset on the watch. In the lower right quadrant you should see a small gold contact on the circuit board with the letters AC (all clear?) stamped on the sheet metal backing next to it. It took a strong magnifying glass for me to see it. Temporarily short the gold contact to the sheet metal backing and the watch should reset itself to 12:00 a.m.
Put the cover back on with the teeny tiny screws and you should be good to go. Be sure to get the o-ring gasket in the channel correctly.