If you are using TurboTax Desktop software, on the File menu (not the File tab) click "New Tax Return."
If you are using TurboTax Online a TurboTax Online account can only be used for one tax return. To prepare another tax return you have to use a different account. If you do not have another account, create a new account. You can have up to 5 TurboTax Online accounts with the same email address. Be sure to keep track of which account you use for each return.
If you prepare another return in the same account, the new return will overwrite the first return, and the first one will be gone and not retrievable.