Dear student,

As per your profile I can assume that you are aam engineering aspirant of class 12th them you must be targeting for jee exams.

Now, for giving jee mains you must have to secure paas your class 12th with aggregate of atleast above 75%.
Now, if you don't have above 75% then you first must give the improvement exam and then sit for jee mains. And after giving improvement exam your best of two scores will be taken into account and then your best score's marksheet still be given. And no previous score well be removed but it will be updated.
Also if you have secured less than 75% in class 12th boards, then you will not get admission in colleges participating in jee mains.

If you will give jee mains then check out the following link to know more about jee mains 2021 :-
https://engineering.careers360.com/articles/jee-main-eligibility-criteria/amp

Hii!
Don't worry. If you are not able to be present for the improvement exam, your marks will be as it is and not be hindered or removed in any way. Next time if you opt for the exam and give the exam, the best out of the two scores will be taken into consideration and you'll be provided a new marksheet.
Hope this helps.
Good luck!