First off, let me inform you that this is only available on the iPhone 6 and up. Here's what you do:
Open your Maps app.
When it comes up, put in a destination. Here I've put in New York, but I've also checked out Dublin Ireland, Madrid Spain, London England, and Rome Italy.
When you get to that map, scroll up from the bottom until you see the bar FLYOVER TOUR. Click it.
Then sit back and enjoy a wonderful 1-2 minute 3D flyover of the location of your choice as it highlights certain destination spots and even identifies them for you!