no dear you can not give improvement exam once again ,you are allowed to apear for it only once in yourlife time.

There are a few requirements that are mandatory if a student wishes to appear in the CBSE improvement exam 2018 for one or more subjects:

• The student must be from CBSE Board
• The student must be passed in all the subjects.
• The student can appear for improvement exam next year only, means you only will appear for CBSE improvement exam in 2018 for the CBSE board exams you gave in 2017 (this year).
• The student must not have any compartments/reappears in order to appear in the improvement exam 2018.
• The student cannot engage in any higher studies in the mean time.
• The student cannot appear in improvement exam for the additional subjects.
• The student can appear for one subject or all subjects in the improvement exam, depending on his will and result.

i hope this was helpful..