You cannot un-file what already has been filed. But you can change it. It seems that you want to Amend the return that you filed. Instead of "Start a new return," look below that for the "Amend a Return" button (next to File an Extension). It seems you should be able to do that before you file your state return if time allows, but I would be careful to check that the right figures are being pulled into the state return, and manually correct if needed.
