These settings affect the time shown in the status bar at the top of the screen, and in world clocks and calendars.
Set whether iPhone shows 24-hour time or 12-hour time: Go to Settings > General > Date & Time, then turn 24-Hour Time on or off. (24-Hour Time may not be available in all areas.)
Set whether iPhone updates the date and time automatically: Go to Settings > General > Date & Time, then turn Set Automatically on or off. If you set iPhone to update the time automatically, it gets the correct time over the cellular network and updates it for the time zone you’re in. Some carriers don’t support network time, so in some areas iPhone may not be able to automatically determine the local time.
Set the date and time manually: Go to Settings > General > Date & Time, then turn Set Automatically off. Tap Time Zone to set your time zone. Tap the Date & Time button, then tap Set Date & Time.