The IRS will accept an amended return up to 3 years after the date you filed the original return or within 2 years after you paid the tax due on the return, whichever is later.
In unusual tax situations, such as net operating loss (NOL) carrybacks, general business credit carrybacks, or situations involving bad debts, you may be eligible for extended amendment deadlines.
However, for the most part, the deadline to amend a 2012 or earlier return has passed.
To amend a tax return for 2013 or later, please see the following FAQ article for instructions. https://ttlc.intuit.com/replies/3288565
