Yes, you can go for improvement to meet the 75% criteria. Usually, boards allow you to appear either for selected subjects or for all subjects again. It is not compulsory to attend every subject, you can choose the ones where you feel you can score better.

For practicals, it depends on the board’s rules. In some cases, practicals are also conducted again, while in others the earlier marks are carried forward.

After improvement, you will get an updated marksheet. This new marksheet will be considered as final wherever the 75% criteria is required. Your old marks remain in record, but for admission and eligibility purposes, the improved marksheet is valid.