I need to add a W-2 can I just go back in and change in lieu of starting all over or amending it? I haven't filed it yet

Have you already filed your return?

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. 

Enter w-2 in the Search box located in the upper right of the program screen.  Click on Jump to w-2.  Add the W-2.

Yes