Changing your stream from science to commerce depends upon various factors. Firstly the college in which you study has to approve for the same. The college looks into account various factors like seat availability, time left for the examination etc before switching streams. Also there has to be a proper explanation for the change. Time is a big constraint for the change as you have to cope up with the new syllabus. Whether you can cope depends upon your IQ level and the time you devote for your studies.
But before changing your stream one thing you should know is that , science field has more future career scope than any other field. Still if you want to change your stream , then Yes, you can change your stream.