If you mean the bezel around the screen, that's not an easy fix and requires you to completely teardown the phone to get at. It typically calls for you to pry the digitizer off the lcd and to pull the LCD off the bezel. Although I have done it, it's not an easy task and you'll more then likely need to replace the LCD when all said and done since its so flimsy and very easy to break. It also requires you to buy 3 torx screwdrivers in specific sizes that are quite hard to find.
There are about 4 well made tear down videos on youtube if you still feel up to it.