Use a thin blade/screwdriver to prise up one of the corners, then work it around. I tend to leave a credit card (not one you need
) in one end, and then work on the other end. Once you prise it out, the pins will hold it in place. Need to be a bit careful as the PCB bends if you go mad with it pulling it from one side. Like Jay says, it's a good idea to get a piece of wood or something to push the pins down while pulling the casing up. You don't always need to though, sometimes it comes out relatively easily.