The calculator comes with a matching protective shield which slides down over the front; when the calculator is in use, you can slide it over the back. This is a nice touch, as it makes the cover much more difficult to lose.
Insert a spudger into the seam between the two halves of the case, and pry it apart.
Watch out for corrosion in older calculators around the battery terminals -- this is why you should never leave batteries in electronics for extended periods of time.