Hey, yes you can change your science stream to humanities after you clear your 11th. You can inquire if your school allow such changes, most of the schools may not allow it because they want to prepare you for your board in 11th and if you change you will have to change the subjects, the environment and miss out so many basics which you will need for your board exams. If you are determined to change and your school does not allow you can change your board to state board which will give you the freedom to change. But first, you will need to clear your 11th exam.

Best wishes