Warranty expires 1 year after purchase of your new phone. If your phone is still under warranty you can go to ubreakifix and they will replace your screen for free if its still under warranty. When you go to ubreakifix tell them that your phone has screen burn and you want your screen to be replaced under warranty.
Screen burn-in is hardware deterioration. Only way to prevent it is to keep your brightness at a respectable level (unless in direct sunlight) and to keep your screen time out low when not in use.