If you want to change the clock on your iPhone’s lock screen so that it always shows 24 hours, here are instructions.
1. Navigate to Settings -> General -> Date & Time
2. Select 24-Hour Time -> ON
Now your lock screen clock, as well as any time functions on your iPhone including World Clocks, Alarms, and the like will all display 24-hour time. To switch back to 12-hour time with AM and PM just tap 24-Hour Time -> OFF in the Settings.