An amended return is used to correct an original return and/or to claim a refund. A properly executed individual original income tax return or an amended return (IRS Form 1040X) constitutes a claim for refund or credit for the amount of overpayment disclosed if the return contains a statement setting forth the amount determined to be an overpayment and advises the IRS whether the amount should be refunded or should be applied as a credit against the taxpayer's estimated income tax for the immediately succeeding tax year. IRC §6402(a), § 6501(c)(7).
The taxpayer should correct his return that is already filed if he finds that:
Documents on IRS.govAmended U.S. Individual Income Tax Return
Your Federal Income Tax
IRS Publication 17