First get a straight-blade screwdriver with a fairly slim blade. From the outside of the case, look at the rectangular backplate and note where it meets the case metal. Slip the tip of the screwdriver between the edge of the backplate and the case metal, and twist a bit while pushing the backplate into the case from outside. You may need to pop it loose in stages. If all else fails you can just get violent with the old one, if you don't need it anymore.
edit: the backplates are installed into the rectangular hole from the inside side of the case, so once you've got the old one out, pop the new one in from the inside. Also, if you haven't already, take the jumpers off JP1 and JP2 to return the memory voltage to normal, as shown
here.