If you look at the FAQs section of DU Admission 2020 portal official website available at https://ug.du.ac.in/index.php/site/faq , question no# 4 says that you can make changes or update information in the application form till the last date of registration. However, there is a correction fees of Rs. 100 that you need to pay towards the changes that you make in the application form. So you can rectify the uploaded marks sheet by logging again into your account. Go to the "Edit" Button at the end of each page or session and make the necessary changes. Since July 4th is the last date for making changes to the application form, please try this out and do  not wait until the last moment. If that doesn't work, please reach out to the help desk at: undergraduate2020@admissions.du.ac.in