When you can file
As long as your original tax return was prepared using Forms 1040, 1040-SR, 1040A, 1040EZ, 1040EZ-T, 1040NR or 1040NR-EZ, then you can amend it using Form 1040X. If your changes affect more than one tax return, you must file a separate Form 1040X for each tax year. Since it may be necessary to prepare additional forms or schedules, be sure you use the ones required for the tax year you are changing.
How you can benefit
When you use Form 1040X to claim additional deductions or credits or to correct an error that reduces the amount of tax you owe for that year, the Form 1040X serves as your formal claim for a refund. Additionally, many small business owners can use the form to carry back net operating losses that occur in one of the subsequent two tax years.
The IRS notes that it takes approximately eight to twelve weeks to process amendments, so be on the lookout for your refund after this time.
Information you need
The most helpful document you need when preparing Form 1040X is a copy of the original tax return you’re amending. You can transfer some of the original information onto the amended tax return such as Social Security numbers, your filing status and gross income. You also need any documentation that relates to your changes. This can be a number of items such as proof of payment for a new deduction you are claiming or an additional W-2 for additional income you need to report. If you attached any schedules to the original return, you don't need to complete these again unless your amendments cause the numbers on it to change.
Time limitations for filing
The IRS limits the amount of time you have to file an amended tax return. For most taxpayers, it must be done within three years of the original filing deadline or within two years of actually paying the tax for that year. However, if you filed the original return after the deadline because you obtained an extension of time to file, then the three-year period begins on the date you actually filed it. Some people who are disabled or otherwise incapacitated may have more than three years depending on the circumstances.