I printed my 2018 return and Turbotax thinks that I have filed. I need to get back into my return and make changes. Is this possible?

Yes, as long as you haven't mailed the return (or e-filed) in yet then you can make changes, please follow these instructions:

1. In TurboTax Online, sign into your account.
2. Select Tax Home in the left menu.
3. Scroll down to Your tax returns & documents.
4. Select 2018, and then select Add a State (you're not actually adding a state, this is to open it back up)
5. Make changes and then file