If you have already filed your return & it has been accepted, you cannot change your bank account information. If your bank information is incorrect, you will be automatically switched to receiving a check for your refund once the banks bounces your deposit back to the IRS & State.
If you have not filed your return yet, go back through the File section, go through the screens until you get to the section where is asks about receiving your refund.
If you cannot get back into your return, see the following
In TurboTax Online, sign into your account.
Select Tax Home in the left menu.
Scroll down to Your tax returns & documents.
Select 2018, and then select Add a State (you're not actually adding a state, this just opens the return back up)