Unfortunately, there is no way to downgrade a TurboTax product once it has been upgraded. You will need to create a new return and choose a lower version. If you enter a donation, the upgrade will happen again.
Once you have started your new return, you can click on"My returns" then "View all tax returns" and click the small Trash Can next to the unwanted return to delete it.
Thank you for choosing TurboTax.