An amended return (1040X) is a form filed in order to make corrections to a tax return from a previous year.
An amended return can be used to correct errors and claim a more advantageous tax status—such as a refund. In order to claim a tax refund, a tax amendment has to be filed within 3 years you filed the original tax return (including extensions), or within 2 years of the date you paid the tax, whichever is later.
File a separate Form 1040X for each tax year you are amending. Mail each form in a separate envelope. Be sure to enter the year of the return you are amending at the top of Form 1040X. The form has three columns.
Important: An amended tax return can be filed even after the tax filing deadline for the tax year has passed.