Your latest marks will be printed in new marksheet irrespective of you get better marks or lesser marks.

Your new marksheet will show the latest marks only irrespective of you get more or less.

Hi

Improvement exams are an opportunity given by CBSE board for its students to improve their previous scores. So if you couldn't improve your score that means if your score was lower or a 'repeat theory' was mentioned in the result then it won't be considered. Your previous mark sheet which has your previous higher score will be valid and considered for your future endeavors.  To again appear for an improvement exam to improve the score as a private candidate is entirely the students choice. SO don't worry your previous original mark sheet could be used for your future admissions.

Hope  this answers your query!