You can amend your tax returns as far back as 2014 if you discover that you have made mistakes on them.  You mean you filed Single when you think you could have filed as Head of Household?

Am I Head of Household?

https://ttlc.intuit.com/questions/1894553-do-i-qualify-for-head-of-household https://ttlc.intuit.com/questions/2900097-what-is-a-qualifying-person-for-head-of-household

 To learn more about amending your tax return, please review the information in the following link.https://ttlc.intuit.com/questions/1908543-do-i-need-to-amend-my-tax-return

Click or tap a tax year for specific amending instructions.

·         2016

·         2015

·         2014

·         2013

Related Information:

·         What does it mean to "amend" a return?

·         Do I Need to Amend my Tax Return?

·         Will amending delay my original tax refund?

·         What is the deadline for filing an amended return?

·         Can I e-file my 1040X to amend my return?