How do i switch the pm to am on my casio f91w-1

SOURCE: Casio F91W-1 Wrist Watch- backlight color

You can change the backlight on a casio wristwatch by holding the Light button, but only if it has an LED light in the side of the screen such as yours does. There is Light Blue, Red, Pink, Blue, Green LEDs in the side of the screen and you can change between them. There is also an option that flashes between all the LEDs when you press the button.

SOURCE: How to change the time on my Casio F91W watch?

first you push the bottem left butten three tmes than push the top left corner to move threw the numbers than use the bottem right butten to change it