@Tanushree You need to start all over again from 11th, with the science stream, and finish off 11th and 12th. But, why do you want to change it? What would you like to do after getting the science course? We might be be able to help you then.
Unfortunately, in India, students can’t switch from one course to another so easily as they can in the US.
Class 12 in India is equivalent to high school in the US. In India, when we pass the 10th exam (called matriculation), students can choose from among either Arts, Commerce or Science stream for their college (11th and 12th). After completing 12th, they then can enter in college to take undergraduate courses. So to get a bachelor degree, one must have completed 12th from the relevant stream (arts, commerce, or science). For example, students who have completed 12th in arts cannot get into bachelor courses in science, biology, engineering, medical, and alike. So, the choice made at 11th and 12th, of choosing from among arts, commerce, or science, is critical.