If you forgot to include some income or to take a deduction or to claim a credit(including the first time homebuyer or repeat homebuyer credit) on your tax return – you can correct it by amending your tax return.
Generally, to claim a refund, you must file Form 1040X within three years from the date you filed your original return or within two years from the date you paid the tax, whichever is later.
Use Form 1040X, Amended U.S. Individual Income Tax Return, to correct a previously filed Form 1040, 1040A or 1040EZ. Be sure to check the box for the year of the return you are amending on the Form 1040X, Line B. The newly revised Form 1040X (Rev. January 2010) has only one column used to show the corrected figures and an area on the front of the form where you explain why you are filing Form 1040X.