Hii Farheen!
Yes, it is possible to Study any stream after you complete your 12th. You can even apply for repeate examination of class 12th. But, in your case, If you want to change the stream, then most probably you will have to take admission in the open Universities providing such courses and complete your 11th 12th and board exam in the respective stream from there. But, this will only waste your years. So, it's better not to cha change streams and continue with what you are doing and put some hard work to perform better. Hope you understand.
Good luck!