Unfortunately, there is no way to downgrade a TurboTax product once you have upgraded it. You would need to create a new return and choose a lower version. You can use any version to prepare all returns, if you start with the Standard version then feel you would like more guidance, you can always upgrade and your information will carry over to the upgraded version.
Once you have started your new return, you can click on "View all tax returns" and click the small Trash Can next to the unwanted return to delete it.
Thank you for choosing TurboTax.