One cannot prepare KVPY in a night or a day in practical terms. You can just go through previous years papers and give tests and accordingly analyse your responses. There is no theory you can go through. If possible go through concepts and main formulas and practice it for a dedicated hour or so.

Know the exam pattern and shortlist few topics which you have to focus in that day and find out all possible questions related to that topic.

That is all you can do in night.

Please don't appear for such an important exam like this. A bit of systematic approach is required.

All the best.