You will just close the software. It might save it automatically depending on which TurboTax software you are using- if Online or Desktop. If using online and once you shut down TurboTax and open it again and if you see it, you will simply leave it or start a new return and once you have completed the "Info" stage of your return  you will be able to click on "review returns" and then you will notice a garbage can beside the one that you do not want to keep.

Thank you for choosing TurboTax.