If you need to change the day on your Casio digital watch, it's a pretty easy process. Here's what you'll need to do:
1. Locate the mode button on your watch. This is usually located on the right side of the watch face.
2. Press and hold the mode button until the display changes. You should see something that says Day or DATE.
3. Use the up and down buttons (usually located on the left side of the watch face) to change the day.
4. Once you've got the correct day, press the mode button again to exit this menu and return to normal time-telling mode.