By the sound of it, your phone is already rooted and has a custom ROM on it, at least that's what usually causes that error.
If you want to update, there are tons of newer custom ROMs to try out, many off them 4.3.1.
If you're feeling really adventurous, you could try 4.4 (there is a build out, just a couple of minor bugs).
Did you buy this second hand or did someone flash a custom ROM for you?