Hello Aayush,

For JEE Mains 2018, you just need to score above 75% in boards. Board marks will be considered just as Eligibility Criteria and WILL NOT CARRY ANY WEIGHTAGE.

If you still want to opt for improvement exams,

• Candidates appearing for CBSE Improvement Exam are issued a new marksheet, reflecting their revised marks. You need to attach it with your previous marksheet.
• Suppose you appear for improvement exams in all subjects and score 90%, and before improvement you scored 95%, then 95% will be considered.
• Yes CBSE updates its database. If you appear for improvement exam as a private candidate, you will be issued a new roll number.
• From the given data, your score 93.5% will be considered.

You may also appear for Open Board exams (NIOS). They offer on-demand examination.

Hope it helps.