If you have made the payment to the application form, then you cannot make the changes. But if you haven't made the payment yet, you can login to your application and click on  "Edit/Change" option on the Application at the end of every page or section. And if you have made the payment, as mentioned, you cannot make any changes to the application form. And if it is Date of Birth error, because it will effect the merit list, you may have to fill in another application form registering with a Different email id and password. But the FAQ's section clearly mentions that no Candidate can fill more than 1 application. But because your application needs an edit in the Date of Birth section, we would suggest you to reach out to the help desk at the earliest. Please mail them from the Registered email id. You can mail them at: pg2020@admission.du.ac.in (mailto:pg2020@admission.du.ac.in) . They might take 24-48 hours to respond. Also go through the Information Bulletin by clicking this link: http://www.du.ac.in/du/uploads/COVID-19/20062020_PG_Bulletin_2020.pdf