Go to Deductions & Credits -> Retirement and Investments -> Traditional and Roth IRA Contributions. Proceed to where TurboTax asks if you want TurboTax to track you Roth IRA basis and answer Yes. Proceed a few more pages where TurboTax asks you to confirm your net Roth IRA contribution basis and adjust the amount so that it is correct. I you also have basis in Roth conversions you can adjust that as well on the subsequent few pages.
