It seems you may not be able to go in and correct the passport number without having it unlocked for you. If the error is in the biographic details, you may notify the consular officer on the day of your interview and take no further action.
Please contact us through our website as we will be required to unlock your case before you can amend the information. If you have made an error when completing the DS-260 you are required to correct the error and re-transmit it if the error concerns the Background and Security questions or passport number. I have made an error on the DS-260, what should I do? Posting this again since it does answer that question