If you are filing a 2020 tax return, you will change your banking information when you go through the steps to file your return. The IRS will most likely use information from your 2019 return. If your banking information is incorrect, you will automatically be switched to receiving a check.
You cannot change a direct deposit for your 2019 return. The IRS may or may not have a portal for tax payers to enter their direct deposit information if they are not going to file a 2020 return. So far no news on how the IRS is going to handle the stimulus checks.
