Unable to update my iPhone 12 Pro to the latest iOS version

—Do you get any error messages while attempting to update? If so, what?

—Are you trying to update "over the air" or while connected to a computer? The latter is often more reliable.

—Are "Automatic Updates" engaged? (Settings > General > Software updates)

—What does the Software Updates screen show below the options? Example:

Have you tried to update your iPhone using your computer as opposed to over the air? You can use a Mac or a Windows Computer. With a Mac, you use The Finder. With Windows, you use iTunes.

For a Windows Computer --> Update your iPhone, iPad, or iPod touch with iTunes - Apple Support

For Mac --> Update your iPhone, iPad, or iPod touch with the Finder - Apple Support

Why not? What happens when you try?