If you have not e-filed or mailed your forms, you can indeed to back and make changes to the original return.
Please go back to TurboTax and under Your tax returns & documents (you may need to click Show to the right, if it is hidden) check that the correct year is highlighted and select that you need to either amend your return or to add a state. You do not actually need to complete either of those, but if you select one, it will open your return back up for editing. Go to the left menu and click on the Federal or State and this will open the federal return so you may add your information and then proceed back to File, on the left menu.
If you have not e-filed or mailed your forms, you can indeed to back and make changes to the original return.
Please go back to TurboTax and under Your tax returns & documents (you may need to click Show to the right, if it is hidden) check that the correct year is highlighted and select that you need to either amend your return or to add a state. You do not actually need to complete either of those, but if you select one, it will open your return back up for editing. Go to the left menu and click on the Federal or State and this will open the federal return so you may add your information and then proceed back to File, on the left menu.