If you haven't made the payment yet for the DU application form, you can make changes to the application form. But if you have already made the payment, then you cannot make any changes to your application form. So we would suggest you to reach out to the helpdesk at: undergraduate2020@admissions.du.ac.in . If the authorities think this will need a fresh registration, then you may have to re-register with a different email id and password and fill an altogether new application. But if the authorities think this is a minor mistake that can be ignored, then you are fine. But eitherways we would suggest you to reach out to the helpdesk for relevant information so that you can take the necessary step. Please do mail them from your registered email id and you can expect a response within 24-48 hours.