Many watches have backs that are snapped on.
Timex is quite good about providing a little tab that sticks out
so that something can be placed under it to pry it off.
Don't try to pry anywhere But where the tab is.
Some have a small indentation instead of a tab.
These are more difficult.
And then there are those with no tab and no indentation,
which can be downright infuriating to get off.
They are also often very difficult to snap back on,
so a "case press" might be necessary.
Presses that look like pliers aren't worth a f**k,
so don't waste any money on such a tool.
Better ones look like an adapted garlic press.
The absolute best are made of cast iron and have a screw like a vise.
They can exert enough pressure to do serious damage, so you have to be careful.
Other cases have backs that are held on by a number of Very tiny screws.
If yours is one, be prepared to keep the screws under control,
as replacements for them are Very difficult to find.
The best (and some junk) watches have backs that are threaded in.
These require a "watch case wrench" to aid in removal.