My watch has a screw- in back on the watch case. It is possible to order a universal spanner wrench for around $10 US. I wanted to make my own spanner wrench so I can replace the battery without needing to get to a store that can change it for me.
In the photo you can see the indents for the spanner wrench so it can unscrew the case back.
Step 1: Use a mending plate
I used a 1/2 x 2 inch mending plate. These are available in any hardware store. Any thin piece of steel would work, too. (The photo is from Google Images.)