IIT

Yes, you can change the branch in second year but for that you need to top the entire batch and branch should also have a vacant seat, usually there are few seats vacant.

Go to Kotak or Indore otherwise Delhi will be good option for preparation of IIT-JEE. These cities have good atmosphere for study.

No, the criteria is of 75% marks for all students.

It depends from person to person. I personally believe that you should definitely drop for JEE. You know you are afraid that you may lower your marks, which has a very low probability of happening. But if you drop and do well next year, you could easily get a government college, which you have always wanted to and let me tell you, my friend, if you give your 100% next year, you shall get CS. You just can't let your fear stop you from trying. If now you let your fear stop you, then you are in a big trouble. Don't even think about marks, just go study.

Hello Pranjal,
Yes, of course you can easily appear in IIT-JEE exam next time to take your score to the high level. You have to only fill the improvement exam form by online mode, on the official website of CBSE. The online applications are normally out for students in the month of August or September every year.

Yes, you'll get some NITs for sure. For your possibilities in IIT, mention your Advanced score.

Dear,
You are to score 75% or more in average or you are to be within top 20 percentile (5% relaxation for SC/ST) in +2 Board Examination. Then with the merit list of JEE Main followed by JEE Advanced, you may be shortlisted and asked for counselling in IITs.

Sorry for breaking your hopes, but the answer is no. For admission in an IIT college, you should have minimum 75% marks in class XII board exams.