I don't see a place to change it on the form either. I don't see anything in the instructions. Here they are if you want to look through them: https://www.irs.gov/pub/irs-pdf/i1040x.pdf
If you enter the explanation for why the 1040X is being filed and include a copy of the revised 1040 with the box unchecked, that may take care of it. It has no affect on the refund or taxes owed since there is no longer a personal exemption for the filer. Sorry I can't give you a definite answer.
