How do I add a 1099-INT after I submitted my return. How do I add this income

If you have already filed your return, you'll need to wait for your tax return to either be rejected or accepted, if it is rejected you can add your Form 1099-INT, correct any other errors on the return and resubmit it.

If it is accepted, you will need to amend your return to include the Form 1099-INT.

Amending won't delay your original refund. However, if you're amending to claim more money, the IRS says to wait until you've received your original refund before filing an amendment. Once you've received your original refund, feel free to cash or spend it while your amendment gets processed

Please be aware that Form 1040X will not be available in TurboTax until February 18, 2021, but once it is available, you will be able to prepare and e-file your amended return.

Please see this TurboTax FAQ for instructions on how to amend your tax return in TurboTax.