Science

You need to discontinue the course and then take transfer certificate if you are studying in any other college under the same university. If you are changing university, you need to get a migration certificate from the university. Go through this process after the admission is confirmed.

This has been already answered and a straight forward answer is no. Without mathematics, most of the institutes won't accept an applicant in BCA, B.Sc IT or CS courses. For more details, visit the respective institute's website and check their eligibility criteria.

Dear,
Through BCA+ MCA programme, you can start your career after 5 years and with your skills, foundation and commitment to technology, you would get the best salary and growth.
B.Sc chemistry is a research-based programme and you need to invest approx. 9 years to reach a good level with Ph.D.
All the best.

You can apply for the entrance tests of various related colleges like JNU, DU, BHU, etc. You can apply on the basis of merit in some colleges, but it will be hard for you as you have secured 81% only and there are many aspirants with more than 90%.