The cbse improvement exam may be scheduled in the month of September from 2nd or 3rd week. You have to apply to your school if you are a regular student then the school will send you the schedule for improvement test. You can check the official site of cbse or you can check the following link to get more information.

https://news.careers360.com/cbse-class-12-improvement-exams-be-held-in-september-2020-check-details/amp

Hii!
CBSE has already completed the application form filling process for rhe Improvement Exams, 2020 for class 10th and 12th. It started in the first week of August, 2020 and ended in the last week of August, 2020. However, the exam is tentatively scheduled to be in September, 2020. There are certain affidavits to be filed and hearings done on the matter of conduction of examination and the number of centres.
Hope this helps.
Good luck!