Unfortunately, once you are past that point, you can no longer change that field in TurboTax Online. You would need to create a new return to change who you are preparing the return for.
To start a new return, select “Start a New Return” from under “My Returns”. Once you have started your new return, you can select “View All Returns” from under “My Accounts & Returns” and delete the unwanted return by clicking the small Trash Can next to it.