If I receive monthly payment child tax credit and believe the amount incorrect for a dependent,how should I go about to correct since tax return was previously filed?

The monthly payment is an advance payment of the child tax credit on your 2021 tax return. Your 2021 tax return will calculate the correct amount of the credit and reconcile it with the advance payments that you received.

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