I initially chose to have assistance completing return but want to change to doing it on my own. Can I do that without having to input personal details again?

No. Unfortunately, once you have upgraded or added features to your TurboTax return, there is no way to downgrade or remove the extra features. You would need to create a new return to go back to the original product. 

To start a new return, select “Start a New Return” from under “My Returns” in the top left corner. Once you have started your new return, you can select “View All Returns” from under “My Accounts & Returns”. Then delete the unwanted return by clicking the small Trash Can next to it or “Delete” under the return’s name.