Can I do 12th again because my Mark's is low.

Hi Vashu

Yes , it is possible to repeat class 12 after completion of your board exams. But I would suggest you not to repeat class 12. If you have cleared all the subjects and you are not satisfied with your marks then you should apply for improvement exams.
You can improve your performance by giving improvement exam.
If you'll repeat class 12 then you will be carrying 2 different marksheets for class 12. So try to avoid this situation and apply for improvement exams. The best thing about giving improvement exams is that you don't have to go to school everyday, just appear for exams and give your best to improve your class 12 percentage.

All The Best!!

U can apply for improvement exam