Unfortunately you can't change an essay once you've got a mark for it. However, if you're unhappy with the mark and believe it's unfair (i.e. you referenced correctly) you could appeal it.
I once got penalised for using footnotes on an essay (I had attempted to get in touch with the tutor to ask about it, and received no reply, as well as there being no advice about it in the style guide) where it was quite crucial to include the info I was putting in them - I then got marks deducted, but went to the head of the department and explained why the use of the referencing was so important to the understanding of the essay (as well as saying I had made attempts to ask whether I could have used them, and obviously if I had been told I wouldn't've) and the deduction was removed.
But yeah, if you just referenced incorrectly and were marked down for it (with no extraneous circumstances), there isn't anything you can do, I think.