If the IRS did not offer to correct your return then you need to file an amended return.
TurboTax can guide you through this and has an option to connect to a Live Tax Expert if needed.
A CP12 Notice is issued when the IRS corrects one or more mistakes on your tax return, and a payment becomes an overpayment, or an original overpayment amount has changed.
-follow these links for additional information-
Understanding Your CP12 Notice | Internal Revenue Service
How to Correct Federal Tax Returns - TurboTax Tax Tips ...
TurboTax® Live - Connect To A Live Tax Expert
