There are tabs on the top and bottom that hold it in. If you push the vents all the way up or down you can see them. I have used a flat screwdriver to lightly pry them up a bit to make it easier to remove them.
Pop the vents out with a small screw driver/blade by putting pressure on the side. with the vents out, the ractangular looking thing will still be in the dash and you will see a bunch of clips holding it in. Pull on the center part, keeping constant pressure and pop up the clips one by one and it will come right out.