The Casio G-Shock line of watches has become a classic brand, featuring a wide array of designs dating back to production that began in 1983. Despite the variety in design, nearly all G-Shock watches can be adjusted in the same way. The only exceptions are the watches that feature "atomic timekeeping" technology, which adjust their own time.
Press the "Adjust" button. It's usually the top-left button, slightly smaller than the rest. When you have successfully pressed the button, one of the numbers on the screen (generally the seconds) should begin to flash.
Choose which number you would like to change (hours, minutes, day, etc.) by using the "Mode" button on the bottom-right to cycle through each option on the main screen. You'll know when you have reached your target position because that number will flash.
Use the top-right button (usually labeled "Start/Stop," or sometimes "Light on/off") to change the number once you have highlighted the position you wish to change. When you have finished making the desired adjustments, press the "Adjust" button again to complete the process.
Use "Adjust" on any screen of your Casio G-Shock watch. To reach the watch's other screens, press "Mode" (bottom-right button). Most G-Shock watches have four screens total, including the main screen. On each screen, use the same process described above to make adjustments to the time (with the exception of the stopwatch screen, which does not keep time.)