I want to file separately from spouse but I checked yes for "I want to prepare our returns at the same time." How do I get back to this screen to fix it?

Unfortunately, once you start the return, you can’t switch from a jointly prepared one to an individually prepared one. You can go to the Return Manager (under Tools), start your return over and choose to prepare your return separately. Then once you have started your new return, you can go back into the Return Manager and click the small Trash Can next to the unwanted return to delete it.