You don't need to file a Form 8606 for a contribution to a Roth IRA. It's a good idea to enter the information in TurboTax in case you take a distribution before age 59 1/2 and need to know the amount of your contributions (basis).
If your backdoor Roth was a non-taxable event, then won't owe any additional tax. You can complete a new Form 8606 showing the revised information and file it with Form 1040-X. Your Form 1040-X won't show any changes to your return. Just indicate in the reason for amending your return is to submit a revised Form 8606. If you have traditional IRAs that caused a portion of your backdoor Roth to be taxable, then you're 1040-X will show changes and result in additional income tax being owed. This will be the case for your state return, if your state has a personal income tax.
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