Hi.

Yes, you can appear for Class 12 again through NIOS to improve your marks. It is valid for most entrance exams like NEET, JEE, CUET, etc., as long as NIOS is recognized by the exam authority (which it usually is).

Make sure to choose the correct subjects and meet the eligibility criteria of the exams you're targeting.
More info: NIOS official site
Details on eligibility for competitive exams: careers360.com/qna