Just received the 8812 tax form how do I change my federal tax form that I've already submitted.

You will have to wait for the IRS to either Accept or Reject your filed federal tax return.
If the return is rejected you can make the necessary changes and e-file again.
If the return is accepted you will have to amend your original tax return. An amended return, Form 1040-X, can only be printed and mailed to the IRS if the original tax return was not e-filed. The IRS will take up to 16 weeks or longer to process an amended tax return.

Before starting to amend the tax return, wait for the tax refund to be received or the taxes due to be paid and processed by the IRS.

The federal amended tax return, Form 1040-X, for tax year 2021 is estimated to be finalized on 02/24/2022

If your return is accepted and you messed up on the entries for the child tax credit, the IRS will likely find your mistake and fix it for you-----but it will delay your refund for a much longer time.  The screens for entering that information warned you that there will be delays if you enter your advance CTC amounts incorrectly.

Thank you. I rushed into doing my taxes without knowing the 8812 form was coming. I opted out of the advances so the my taxes aren't that far off. Just reads $0 on line 14f instead of $250