The TurboTax online web-based editions can only be used to complete one tax return per account and User ID. To start another tax return using the online editions you will need to create a new account with a new User ID.
Go to this TurboTax website and click on Sign up in the upper right of the page to start a new account - https://turbotax.intuit.com/personal-taxes/online/file-your-own-taxes/
