If you need to make a change or adjustment on a return already filed, you can file an amended return. Use Form 1040X, Amended U.S. Individual Income Tax Return, and follow the instructions. Form 1040X cannot be filed electronically.

You should amend your return if you reported certain items incorrectly on the original return, such as filing status, dependents, total income, deductions or credits. You may also amend your return if you originally claimed the first-time homebuyer credit for a 2009 home purchase under the prior law allowing a credit, which generally must be repaid, of up to $7,500 rather than the newly expanded credit of up to $8,000 that normally doesn’t have to be repaid.

However, you don’t have to amend a return because of math errors you made; the IRS will correct those. You also usually won’t have to file an amended return because you forgot to include forms, such as W-2s or schedules, when you filed — the IRS will normally request those forms from you.