Once your return has been accepted, you can amend it. How you tell TT that you want to amend a return probably depends on which version you are using. For the Premier version I use, there is a button on the page that lets you continue a return or start a return and below that a button that says "Amend a filed return". TT will walk you through the process of creating the 1040X. Note that the amended return must be mailed and cannot be e-filed.
