How do I go back and change my tax returns once it's already been accepted by both state and federal?Child tax credit got messed up and it didn't claim my other daughter

[SteamTrain]

IF that child became 17 during 2019, then that is probably correct.

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Otherwise...you must wait for your original refund to be issued (or pay the original Balance due).

.....then in late Feb, log back into your account and start the special AMEND process...make your edits and any other changes needed...then print out the amendment forms and take them to the Post Office and mail them in.  They will take a couple months to get processed, since they are manually processed.

[LeonardS]

Because your return was accepted by the IRS  you will have to file an amended tax return using Form 1040X to correct your tax return.  

This link to the TurboTax Help page  How to amend (change or correct) a return you already filed provides information on how to amend your tax return.