Hi Kavita,

If you want to reappear and pass class 12 th again, you have to reapply for the exam through your previous school of any school which has the board you want to give your exam from. If you want you may also attend classes in that particular school because syllabus might have changed by now, it would help you in studying as well and help you score good as well. Rest of the procedure will be briefed to you by the school authority itself. Please carry a passport sized photograph with you.

Hopefully this helps.