As long as you have not Efiled or mailed the return, you can go back in and make changes. Please see the steps below. These are from last year so I don't know how it works for 2023.
In TurboTax Online, sign in to your account.
Select Tax Home in the left menu.
Scroll down to Your tax returns & documents.
Select 2023, and then select Add a State (you're not actually adding a state, this just gets you back into your return so you can make changes).
I tried that approach 20 minutes ago and it did not work. No tax-part menu on the left. But I just tried it again and it worked! Thanks for the tip, which I already found elsewhere. I think a lesson here is that one may need to wait for 30 minutes or so while TurboTax sorts out that a "Ready to mail" tax return should still be editable.