Federal Income Tax Return
If you made a mistake on your tax return or forgot to claim a credit or deduction, you can fix the return by filing an amended return. You should amend your return if you need to correct filing status, the number of dependents you claimed, or your total income. The IRS generally corrects math errors when processing your return and will send you a letter if you forgot to include a schedule or other item.
If you have to file an amended tax return because you realized that a previous return was not filed correctly or you received corrected forms W-2, 1042S, etc., you usually have to use Form 1040X (see for instructions here). For a short introduction on what is needed see this IRS Article.
If you have to amend more than one year, mail each year's amended return separately.
In terms of timing, as of the IRS website: "The Form 1040X should be submitted after you receive your refund or by the due date of the return, whichever, is earlier." To claim a refund, you generally must file Form 1040X within three years from the date you filed your original return. You can also file it within two years from the date you paid the tax, if that date is later than the three-year rule.
To prepare the Form 1040X, you are required to show what was on your original return, show the amended amounts and give a reason for the change.
Be sure to attach the following to Form 1040X:
1. A copy of the original tax return marked "copy of original return" at the top;
2. A tax return correctly filled-in and marked "amended return" at the top.
These two attachments will show where the numbers are coming from on Form 1040X.
If you received any tax forms (W-2, 1042-S, 1099MISC, etc.) after you filed the original return, attach these "new" forms as well.
To complete the "amended return", go to the IRS website, type into the search box on the upper right side "tax forms for XXXX" (xxxx = year for which you need to redo your taxes). This will bring you to a link that has all forms and publications for that year. You will need the tax form for that year, i.e., Form 1040, 1040EZ, 1040NR or 1040NR-EZ and its instructions. Fill out this tax form per instructions with the new information. E.g., you realize that you can claim treaty benefits for your financial aid, enter here all aid as treaty exempt. Or you can claim a treaty benefit on your wage income, enter here the amount that is left of your earnings after you subtracted the treaty amount you can claim.
Mail the Form 1040X to the appropriate address (see Form 1040X Instructions). For nonresident alien tax returns it would be the Department of the Treasury, Internal Revenue Service Center, Austin, TX 73301-0255.
If you need a copy of your old tax returns, you can call the IRS. They will send you transcripts.
State Income Tax Return
If you have to file an amended Federal Income Tax Return, you generally also have to an amended State Income Tax Return. You will have to included a copy of the amended Federal Income Tax Return alongside the amended State Income Tax Return.
For further instructions see by state: