Hello antra

If you are in 11th class you can change your stream even after registration, you can talk to your principal, write an application letter for changing stream with your parents sign on it and submit it to your school administration, as 11th class is independent of 12th class, so you don't face any problem.

Only choose national institute of open schooling if your school doesn't change the stream but that's very rare case.

I hope it helps.