How can I get the back off a watch?

You actually don't need any special tools to open a screw on back. There should be special grooves on the back indicating that it's screw on instead of pop on. I use a pair of needle nose pliers ! Using the needle nose pliers straight up and down on top of the case,put the tips into the groves on either sides. Be careful to not scratch by slipping! U can use a thin peace of cloth over the back while doing this for a little protection. But that has always worked for me. Depending on how tight it's on May need quite a bit of force to break it free. I hope this helps ! Good luck ...

If you have a snap-on back you can use a knife blade or razor blade[careful] to pry/lift the back off. The back normally has a slight bevel [hard to see] where you would insert the blade to pry….

Harbor Freight also sells an inexpensive adjustable wrench for watch screw-backs, hopefully your batteries just press in and don't need other tools...