Great question! Unfortunately, it is damaged now unless you're willing to keep it that way.
A screen "Burn In" happens when your device gets too hot, or is sat in direct sunlight while the screen is on. It can also happen if you leave your iPhone on one screen for an extended amount of time. Another example would be, if you had a TV and left it on its Home Screen for multiple days, it would do the same thing.
Only way to fix it would be to have it sent to Apple to have it fixed, if you paid for the AppleCare, it shouldn't be very much since you're already covered.
If not, it would be full price for the job which wouldn't be worth it more than likely, unless it bothers you that much and you plan to keep it for another year to 2 Years.
Best Of Luck!