If you have passed your practical exam but failed in your theory exam, you have to reappear for your theory exam in the improvement exam. The practical marks will be carried forward.
Hope this helps you.
The improvement exam is for those students who have got bad marks in some subject /or if you want to improve your marks in that subject. So you have to fill the improvement form .
So next year you have to give the exam along with your Juniors. You register as a private candidate and you are alotted a new roll number. You get the same Paper as your Juniors get.
You don't need to give Practical Exams again, though. Their marks are carried over. You have to give theory exam only.
Hope it helps.