If you have not mailed out the original return yet, you would still be able to go back into your original return to fix it and reprint. It would not be considered a Amended return because nothing was sent to the IRS yet. You can cancel this by following the TurboTax FAQ below:
How do I cancel or undo an amended return in TurboTax Online?
Then to get back into your original return to make the change please see the instructions below:
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In TurboTax Online, sign into your account.
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Select Tax Home in the left menu.
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Scroll down to Your tax returns & documents.
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Select 2018, and then select Add a State (you're not actually adding a state, this just gets you into the right menu).
Once you have made the necessary changes you can reprint and mail your return to the IRS.