I printed returns to mail in - "filed" them in TurboTax's eyes - then caught a mistake before filing with IRS. How do I fix that in Turbotax w/o "amending" my return?

Got an answer to this! Simply go to "add a state" and that puts you back into the correct place.

When you sign onto your online account and land on the Tax Home web page, scroll down and click on Add a state.  See screenshot

This will take you back to the 2018 online tax return to make your changes before filing the tax return.