If you are using TurboTax Online, then you would have to start your return over if you want to switch to preparing with your spouse/partner.
To start a new return, select “Start a New Return” from under “My Returns” in the bottom 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.