If you are already working on your return and are in the File tab, you can go back to make a change by clicking either the Personal Info tab, Federal Taxes tab (Personal depending on version), or the State Taxes tab depending on where you need to make a change. Once you have made the change you can navigate back to the File tab to complete your return.
If you are on the Tax Timeline, you should be able to click on the orange 'take me to my return' button. If your status is 'ready to mail' and you haven't mailed your return yet then you can go to My TurboTax > Tax Timeline > some things you can do > Add a State to get back into your return.
