Many mistakes are corrected in processing and a letter of explanation is mailed at the time the refund is issued. If the mistake was not corrected in processing, you need to file an amended or corrected return using Form 1040X (PDF), Amended U.S. Individual Income Tax Return, as soon as possible. Include copies of any schedules that have been changed or any Form W-2 (PDF) you did not include. If you return the refund check with a letter of explanation, a refund in the correct amount will be issued when the amended return processes.

References:

• Form 1040X (PDF), Amended U.S. Individual Income Tax Return
• Tax Topic 308, Amended returns

Source: www.irs.gov