Assuming its the Defender case you have, you first have to remove the rubber housing. Try starting from the bottom, where your charging port is. It peels back, and over the clips on the sides and top.
Once it is off, start unclipping from one side, and working your way up and around the shell. It should open up then.
Installation is basically the reverse of what i said. Be sure to clean your screen, and the plastic screen on the case before putting everything back together.