Mistakes happen. You file your tax return, then realize you forgot to report some income or claim a certain tax credit. You typically don't need to redo your whole return. Along with filing an amendment using Form 1040-X, you’ll also need to include copies of any forms and/or schedules that you’re changing or didn’t include with your original return.
If you’re filing a Form 1040-X to correct an error on your original return, you’ll need to file within three years after the date you filed your original return.