If you want to do again your 12th class with different subject from the same board you can do it but you have to register freshly with the subject of your choice. Now I would recommend that if you want to repeat your class 12th then go for open schooling it would be better for you. Now if you want to do from regular board then you can go to your school and get it discussed the officials will guide you regarding the process.

I hope this answer helps. All the very best for your future endeavors!