You can prepare an amended return online, but you cannot file the return online, explains efile.com. Amended returns must be filed by mail to the IRS. Returns do not need to be amended for small math errors, as the IRS may find and correct those errors for you.
When filing an amended tax return, you should use Form 1040X. You need to enter the year at the top of the return which you are amending. You can generally file an amended return three years from the date you filed your original return or two years from the date that you paid your tax, whichever is later, according to the IRS.
If you are amending more than one return, each year requires a separate return mailed in a separate envelope, and if the change involves another form or schedule, you must attach the form or schedule to your amended return. If you anticipate an additional refund, you need to wait until you have received your original refund before you file your amended return, and you can cash any refund check you receive. Any additional tax owed should be paid before the due date to limit the amount of interest and penalty charges you may incur, explains the IRS.