Review the FAQ's for the Advance Child Tax Credit on this IRS website, especially questions for eligibility in Topic A and receiving the payments in Topic G - https://www.irs.gov/credits-deductions/2021-child-tax-credit-and-advance-child-tax-credit-payments-frequently-asked-questions
If you are eligible go to this IRS website to update your bank account by clicking Manage Payments - https://www.irs.gov/credits-deductions/advance-child-tax-credit-payments-in-2021