Hi Aspirant,
For the students who are not satisfied with their marks in 12th, cbse offers the choice of appearing in improvement exams.You can appear in any number of subjects you want but have to make sure that you can only appear in the subjects in which you have passed . Improvement exams can only be given after passing in class 12th.
The process for the same is you need to fill up the application form of the exam which can be collected from the regional offices of cbse or in the syndicate banks.Then you apply for the subjects and pay accordingly.
You can also fill up for the form online by going through the website of cbse.You have to fill in the required details and pay either online or through e-challan.
And after the successful submission of the form,you become eligible for the exam.
Hope this helps.