Remove the 3 screws in the wheel wells behind the rear tires; one of them points upwards and is kinda hidden. Then use a flat screwdriver to pop the center of the two round plastic clips on the underside, and remove the clips completely.
Then tug on the sides of the bumper where they meet the wheel wells to pop them out of the brackets. Don't be afraid to use a little force, the bumper cover is pretty flexible. Finally pull the bumper cover backwards away from the car, it should slide right out. Don't forget to disconnect the license plate connector as you do this.
Installation is essentially the reverse, start from the middle and make sure it's perfectly centered/aligned before you push it back in. Make sure the top edge properly slides into all the retaining clips and doesn't go over or under them. Pay particular attention to the clips under the taillights and make sure the bumper goes all the way into them. Then when it's lined up snap the sides back in, reinstall the screws clips and connector and you're done.
A word to the wise: before reinstalling the bumper cover, squeeze all the retaining clips that go all along the top edge so that they are tighter and will grab the bumper better. That way you're bumper won't be popping out of place later as some of us have experienced. You can use a flashlight to examine the retaining clips from underneath the car and make sure the bumper is installed properly.
