Your banking information cannot be changed after your tax return has been filed and accepted.
Banking information does not go from TT to the IRS until you file your 2020 tax return and it is accepted. The IRS will begin to accept and process 2020 returns at the end of January 2021.
You enter/change banking information in the FILE section before you file your 2020 return. Changing it on the TurboTax site does not do any good until you file your return and the IRS processes your return. If they manage to start sending out stimulus checks before you can change your information, we hope the IRS provides a way for you to change it with the IRS. If your check goes to a wrong or closed account, it will be sent back to the IRS and then the IRS will mail a check to the address on your tax return.
this is the IRS FAQ on the subject.
No. To help protect against potential fraud, the tool does not allow people to change direct deposit information already on file with the IRS.
If we issue a direct deposit and the bank information is invalid or the bank account has been closed, the bank will reject the deposit. We will then mail your Payment as soon as possible to the address we have on file for you, and we will update Get My Payment to reflect the date your payment will be mailed. Typically, once the payment is mailed, it will take up to 14 days to receive the payment, standard mailing time.