Amend will not be available untill the IRS releases Form 1040X. We anticipate that to be in mid to late March. When you amend in TurboTax it does not let you directly go to the state. You walk through the amend screens until you come to the state questions. You do not have to make any changes on the federal screens, but just click through them until you get to the section that you need to amend.
When you are done with the amend, you will have the option to only send the state amend.
You have 3 years from the date you filed your return or 2 years after you paid the tax due (whichever is later) to file an amendment.