In case you lost the manual for your Casio watch

Use this page to download a copy of CASIO watch manuals in PDF format. To download a PDF file, enter the four-digit (or three-digit) module number marked on the back cover of your watch. There are pics to show where the module number is found.
Timepieces Watches Manuals CASIO

You can find the manual for your watch by filling in the module number of your watch (that is the 4-digit number in a square you'll find on the back of your watch case) in the search box here:
Timepieces Watches Manuals CASIO

You can find the manual for your watch by filling in the module number of your watch (that is the 4-digit number in a square you'll find on the back of your watch case) in the search box here:
Timepieces Watches Manuals CASIO

Since you don't state which specific Casio watch you own, you can find the manual for your watch by filling in the module number of your watch (that is the 4-digit number in a square you'll find on the back of your watch case) in the search box here:
Timepieces Watches Manuals CASIO

Actually, the watch has 3 buttons -- "Light" being the 3rd one. Press Mode until a number in the time mode begins flashing. Then press the Light button to get it to the hour indicator. Use the 12-24 Hr button to set it to the current hour. Then press the Light button again to get it to the minutes section. Again, use the 12-24 Hr. Button to set it to the next minute coming up in the current time. Then push the Light button again. This will bring up the seconds indicator. At the precise time the minute you set before begins, hit the 12-24 Hr. button, and the indicator goes back to zero and begins ticing away. Then hit the Mode button again until the watch is in the current time mode. Hope this helps. I use Internet time to set a precise time.