I don't know what you mean by "manually update".
If the child is correctly listed as a dependent, then when you are in the federal review section, you enter the amounts you actually received and the extra will be added to line 30.
If you went to the review section and the program is not adding the extra, then you have entered the dependent incorrectly. You need to fix the dependent, rather than manually entering the credit. Making a manual entry requires doing an override in the desktop program (can't be done online) and the override will void the accuracy guarantee. The IRS will remove it anyway, if your return doesn't show you qualify. You may have a dependent problem you need to fix instead.
Also note, if you received a payment based on claiming your child "Adam" in 2019, and in 2020 your ex claims Adam and you claim your other child "Betty", you don't get a second payment. The credit is based on the number of dependents you claimed, not who they are.
