The 1040X form is available on the IRS site, but not yet incorporated into TurboTax software. When TurboTax is updated in late March, then the software will do the work of fillilng out the 1040-X as well as other forms that may change.
You generally must file an amended return within three years of the date you filed the original return or within two years after the date you paid the tax, whichever is later.
