There's something official about filing tax returns that makes them seem set in stone. But tax returns are amendable - and not just by the IRS. Just as the IRS can reach out to you to check on certain details or request certain forms, you can also amend your tax return to better reflect your financial situation.
Why would you amend the return? In some cases, you would do it to avoid the risk of an audit or penalties if you know you made a serious error. But many people amend returns in order to claim a deduction that they forgot or otherwise failed to take advantage of.
Let's say that you bought your first home two years ago and paid a good bit of interest on the mortgage. When you filed the following April, you maybe didn't realize that you could deduct mortgage interest, and overpaid as a result. You could file an amended return to reflect the omission, and would likely be entitled to a refund.
How long do you have? Generally, you must file an amended return (Called a 1040X) within three years of when you filed the original return. If, for some reason, you paid the tax after that time, you would have two years from the date the tax was paid.
Taxes can be complicated, especially when trying to dig up records from several years ago. Therefore, you might find it beneficial to work with a tax professional such as a certified public accountant. And of course, if you find yourself facing an audit or other scrutiny from the IRS, please seek the help of an experienced tax law attorney.