If you have already filed your tax return (either efile accepted or return mailed) you will need to amend to make changes.
If your return is rejected, you can simply make your changes before you re-submit your tax return.
You should wait until your already-filed tax return has been processed (i.e., refund received or payment deducted from your bank account). Then, you can use TurboTax to prepare an amended return.
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There is nowhere on your 2019 return that you were required to enter last year's additional tax credit amount.