If you need to make a change or correct your federal tax return after it has been filed you will use the Form 1040X.

As a general rule, a tax return is deemed filed on-time if it was mailed and postmarked on or before the due date of the return. (In this day of electronic filing this is less of an issue). It is also a general rule that a taxpayer has three years (including extensions) after the date they filed the original return to file an amended return.