Where do I find the section to choose to use old or new rules of tcja? When I filled out my 12yo form initially it asked which rule i wanted to use, now i can't find it

If you delete the form to re-enter, you can start fresh.

To Delete a Form

Open your return in TurboTax. 

• In the left side bar, select Tax Tools> Tools.
• In the pop-up window Tool Center, choose Delete a form.
• Select Delete next to the form/schedule/worksheet and follow the onscreen instructions