There is no way to correct the payment amount you were sent in advance of filing the 2021 return where the credit will be reconciled. Remember the credit has 2 phase out calculations so you may not get the full $250 or $300 as they are advertising on the TV. https://www.irs.gov/credits-deductions/advance-child-tax-credit-payments-in-2021
Did you not get the advance notices explaining what to expect ?
https://www.irs.gov/individuals/understanding-your-letter-6416-or-letter-6416-a
