1. Press [A], the top left button, and the seconds should start flashing
2. Once you have some digits flashing, press [D], bottom middle button, and will switch between the 12h/24h formats
3. Wait a few seconds and the digits should stop flashing and return from the time set mode, to timekeeping mode.
Here is a link to a manual directly from Casio, showing you all of these steps.
As per the explanation on top of the manual, this should work with 3 different models of Casio watches, including yours.
http://ftp.casio.co.jp/pub/world_manual/wat/en/qw1659.pdf
Have a good day