No, it is not necessary to give practical exam again. If you passed and have get good marks in practical,then you can only give theory exam to improve your score. The previous practical marks will be carried forward and accounted for and students will appear in the theory exam only.

When taking an improvement exam for a subject in NIOS with practicals, you only need to sit for the chemistry theory paper if you passed the practical component already as NIOS allows you to re-take only the theory portion of a subject if you wish to improve your marks, while keeping your existing practical marks if they were satisfactory.

Further NIOS states that If the student passes in practicals but fails in the theory of a subject, his/her practical marks will be retained and he/she can appear again ONLY in theory papers.

I hope it is clear to you!!

All the best for your exam!!