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. Right now there are no new guidelines. 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.
If the IRS does not already have direct deposit information for you, they will send your second stimulus check by mail. There is no way to provide direct deposit information if they do not already have it.
See the following IRS News Release for more information about how and when the second stimulus payment will be made.