Hmm... typically you simply replace the rear bumper "cover", I'm not really sure what this "apron" terminology is. Is the apron really the lower rear "spoiler" or trim piece that runs along the bottom of the rear bumper?
A bumper consists of several parts, the outermost part being the cover (typically just a plastic molded piece that's painted). Under that is usually some foam and then the real "bumper" -- a metal impact bar or plate which is there for those > 5mph collisions.