To delete your return, you would have to start a new, blank return, as you can't delete the most recent return you were working on. To start a new return, select “Start a New Return” from under “My Returns” in the top left corner. Once you have started your new return, you can select “View All Returns” from under “My Returns”. Then delete the unwanted return by clicking the small Trash Can next to it or “Delete” under the return’s name.
To delete your account entirely, please go to https://accounts.intuit.com/ and login. Then select "Data & privacy" from the left side menu. On the Data & Privacy page, click on delete. Then you'll get the option to "Make A Delete Request".
Note: If you delete your account, any returns you had in TurboTax Online will be permanently deleted.