Based on what I see and read at https://www.irs.gov/credits-deductions/advance-child-tax-credit-payments-in-2021 and my understanding of it, I would recommend that you do nothing and not amend your 2020 tax return. You'll be able to make up the difference when you file your 2021 tax return, based on what I read and my understanding of the information on the aforementioned web site.
