My initial reponse would be to simply suggest you either 1.) RAISE your seating (add a cushion, or the proverbial phone book) or 2.) place the IMAC on a lower surface top!
I'm giving your query the benefit of the doubt (in that there MUST be more details involved or you would've done either of the above by now on your own). I can only suggest you pursue a better location set up for your iMac, as it would be far easier than wracking your brain over trying to figure out how to modify the actual system/display (in addressing a problem too easily resolved by common sense means...
In all honesty, I am still trying to wrap my brain around the possibility your query was, in fact, serious, and not the "you've got to be kidding us, right?" joke it resembles... Apologies if I've misunderstood, but I only have your own (question) words to go by...
Good luck. ;-)