A. If you discovered a mistake in a tax return that has already been filed, you can amend it by using 1040X and the appropriate forms. IRS has clear rules about amendment.
Deadline to Amend If IRS Owes You Money
For amended returns, the IRS sets a deadline of three years from the date the original tax return was due. For example, if the due date of the return was April 15, 2010, you have until April 15, 2013, to file a Form 1040X for 2009.
If you paid the tax after you filed the return, then the deadline is two years from the date of your payment if that date would be later.
Deadline to Amend if You Owe IRS Money
If you found that you made a mistake and owe IRS additional tax, while the IRS will correct your tax return if it has the information supporting a change in your tax liability, if it does not have that information, you should file an amended return as soon as possible and pay the additional tax before the due date. If you don't get the additional tax payment in on time, the IRS might levy penalties and interest on the outstanding balance.