Online mode have many advantages so offline students will be at a disadvantage , so it won't be fair right , why do they have two modes which are not fair ,( I don't know to express my views any more , I hope you understand what I mean)

Okay now I understood your question.
YES. As you mentioned CBE (computer based Exam) do have a little advantage. You can change your answer option any time. But a student is given both the options equally. Some students are not comfortable with the virtual format test. Otherwise there no ground breaking advantage.

How to change my mode.( I have completed all the process )

Once filled the form it's over. You can't change this option.

You haven't mentioned which mode you selected. If you let me know I can help you

I have chosen offline mode, can you help me with this ,( I am studying state board syllabus ,and the gap between the board exam and jee mains offline is just one day)

I have studied both 11th and 12th , through a coaching center , another problem is that I don't get time for revision , what should I do ?