I have not filed my return and need to update the forms in turbo tax. The only option I have is to click "Amend (change) return?

From my colleague DoninGA,:  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 below). 

You will not be adding a state.   This just allows you to go back into the program.

This will take you back to the 2018 online tax return where you can make changes as needed