Yes, if you have studied Arts in 12th, then you can also study Science later. For this, you will have to appear for science exam. Also, your Arts mark sheet will also remain valid . However, admission cannot be taken simultaneously in two courses affiliated to the same university.

After studying in Arts and Science, you can go into different fields. After studying in Arts, you can go into fields like journalism, advocacy, social service, arts. Whereas, after studying in Science, you can go into fields like engineering, medicine, research.

Hope this is helpful to you!!