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:

1. He did not report some income;
2. He claimed deductions or credits he should not have claimed;
3. He did not claim deductions or credits he could have claimed; or
4. He should have elected a different filing status.

Resources

Documents on IRS.gov

Amended U.S. Individual Income Tax Return
IRS Form 1040X

Your Federal Income Tax
IRS Publication 17