Form 1040X

• Use Form 1040X, "Amended U.S. Individual Income Tax Return," to file corrections for any version of Form 1040, such as 1040A or 1040EZ, that you previously filed. You cannot file Form 1040X electronically and must submit the form by mailing a copy on paper to the Internal Revenue Service. You can file an amended form the same year as your original submission or up to three years later.
  
  

Missing W2

• If you filed your taxes and did not include a copy of a W2 form with your submission, you do not need to submit a corrected return as long as the original filing reflected all your income. If you did not include a source of income on your original return, regardless of whether the income was reported on a W2 or some other form, you should file an amended return on Form 1040X.
  
  

Mistakes Not Requiring an Amended Form

• You do not have to file an amended return to correct simple arithmetic errors on your original tax return. The IRS will check the arithmetic on your return and notify you of any corrections that are needed. You do not need to file a corrected return if you didn't submit a required attachment, such as a copy of your Schedule A (itemized deductions) or Schedule C (business income). The IRS will request copies of these documents if they're needed.
  
  

State Tax Forms

• You should check with your state tax agency if you file a federal 1040X to determine if you also need to file an amended return with the state. States generally have similar rules to the IRS regarding amended filings, so a federal submission often requires a state submission as well.