Check the appropriate box in the Filing Status section under the taxpayer name and address on Form 1040-X.  Explain on Page 2 why the mistake was made on the original return.  

Make sure the PTO version of the return matches what was filed as a single person, then follow the steps given here https://proconnect.intuit.com/community/help-articles/help/how-to-amend-an-individual-return-1040x-o...