What can I do to improve my class 12 marks if I was absent in CBSE improvement exam
As of now in this year you can not improve your marks and not even in the next year.You will get old marks that you had scored in class 12th board exams and your improvement exam for that subject in which you were absent will not be considered.
Try to apply for colleges through your class 12th board exam marks according to your interests.
Good luck.

You need to be satisfied withyour old scores and start applying to colkeges which accept 45% marks aggregate for engineering.
If your score is above 75 %aggregate only then you qualify for IIT and NITs.
Ekse you can try out tier 2and tier 3 engineering college.