If you have not already filed: TurboTax will ask you for your direct deposit information under the File tab once you have completed and reviewed all of your tax information. To access or change your banking information in TurboTax Online you would go to the File section. See this FAQ for more details:
If you have already filed:
Once your return has been accepted (or mailed), it's too late to change your bank account for direct deposit. If the info you provided is for a closed or invalid account, the government will just mail you a refund check.
If the info you entered happens to be someone else's account, don't worry – banks won't deposit tax refunds into accounts where the names don't match up. You can contact your bank and alert them that you have made an error in the account number.
You can review the direct deposit account information you entered by looking on your printed return. This will be found on your 1040 under the Refund section (look for line 76).
Now, if your recently-filed return is in pending status, wait until it's either accepted or rejected. If rejected, simply open your return in TurboTax and go through the File section again to update your bank account before re-submitting your return (don't forget to also take care of whatever problem caused the rejection).
If accepted, you'll get your refund check in the mail.