If you have a return in which you believe you are owed a refund from the IRS you cannot delay in filing it to claim the refund. Strict time limits apply to the filing of these returns and they must be filed within 3 years of the original due date of the return. If you miss this deadline you are forever barred from claiming the refund.
If you believe you have overpaid your tax, you have a limited amount of time in which to file a claim for a credit or refund. You can claim a credit or refund by filing Form 1040X. See Time for Filing a Claim for Refund, above.
File your claim by mailing it to the Internal Revenue Service Center where you filed your original return. File a separate form for each year or period involved. Include an explanation of each item of income, deduction, or credit on which you are basing your claim.
Corporations should file Form 1120X, Amended U.S. Corporation Income Tax Return, or other form appropriate to the type of credit or refund claimed.