hi,

see in any of the board exam there is separate exams take place once the results has declared for those who was unable to clear the exam or need to improve the score obtained in the 12th course. generally once the results declared examination for improvement or supplementary takes place with in three month. so you have to fill the application form for the required exams by online or offline, but make sure you have the well prepared for the exam, since the subjects you would apply for improvement, if your number decrease this time the current number would be considered and you will get a separate marks certificate mentioning the improved subjects score.

I suggest you to research thoroughly exactly which subject you want to give for better marks achieving, study hard , all the best for your future endeavor.