Hello Monika,

you can ask your school authorities to help you or you can directly go to your school's T.C. office and get the work done.

In most of the schools, before we  are appearing for the exams, the school circulates us a form where in we can correct the errors(if found) . You can wait for this to happen in class 11th or can also wait till class 12th. after applying the changes , you can then download your updated marksheet from the cbse portal.

if you want to do the changes made as soon as possible, you can send a mail to the cbse office and then wait for their reply.

Hope that helps

All the best.