Hello Manish!

To answer your question, yes you are eligible to change your stream to commerce field even if you had passed your 12th boards with science stream. All commerce related courses are open to science students, however some courses do require maths as a prerequisite to be eligible to apply for admission. Mathematics will be required in courses like B.Com(Hons.) and related courses. Delhi University requires a student to have passed in maths in boards to be eligible for B.Com(Hons.). Although for B.Com(p) there is no such requirement in DU. IP University, for instance don't require maths as a prerequisite for B.Com(Hons.). The admission to these courses in various universities and colleges are held either through entrance exam or on merit basis. I recommend you to research a bit on these courses before getting admitted since Commerce field would get you acquainted with subjects like accountancy which will again require your solving abilities.

All the best!