The reason may be the "24-hour format" option under your phone's Date & time settings.

If you swipe down from the top portion of your phone display while it's on, this will make the Notifications screen show up. On the top-left corner of this screen is the time and date, and tapping on it will make the Date & time settings screen to appear. This is where you'll find the 24-hour format option.

Have you checked to see if this really is the cause of it?

The reason may be the "24-hour format" option under your phone's Date & time settings.

If you swipe down from the top portion of your phone display while it's on, this will make the Notifications screen show up. On the top-left corner of this screen is the time and date, and tapping on it will make the Date & time settings screen to appear. This is where you'll find the 24-hour format option.

Have you checked to see if this really is the cause of it?