You can clear and start over as long as you haven't paid or registered yet:
Sign in to TurboTax Online and open your return by clicking the orange Take me to my return button.
With your screen maximized, click My Account (at the top of your screen toward the right) and select Clear & Start Over. (If on a tablet or phone, you may find the Clear & Start Over option by clicking the three parallel lines, or “hamburger”, in the top left corner.)
Answer Yes to Are you sure you want to clear your return?
After your entries have been cleared out, we'll prompt you to
select a TurboTax product so you can start over from the very beginning.
You have clicked a link to a site outside of the TurboTax Community. By clicking "Continue", you will leave the Community and be taken to that site instead.