Hii aspirant..

Generally there is no option being provided to students for changing the subject in the improvement examination.

As you have not mentioned the specific School Board or the state board that you are talking about I cannot give you the exact detail.

But improvement exam is generally taken for the students who have failed to get good marks in any particular subject in their board exams and if you are failed and did not get good marks in a subject then how can you expect to give the improvement exam in any other subject.

Thus you are not allowed to give improvement exam in any different subject than the subject you have failed to get good marks.

Hope it helps. 

Best of luck.