Since you did not specify State in your post, I assume you are referring to starting another online tax return.
The TurboTax online editions can only be used for one tax return per account and User ID. You will need to create a new account with a new User ID for another tax return. Click on Sign up at the top right of the following website to start a new account - https://turbotax.intuit.com/personal-taxes/online/file-your-own-taxes/
