Hey Manish !
Ofcourse you can appear for the CBSE Improvement Exam 2020 considering the fact that your result of 2019 wasn't satisfactory.
You can apply for all the major subjects, theory or practical. The discretion is based on your requirement though.
The online forms are available now and you can go through the site mentioned below to know more about the rules and regulations of the Exam.

http://www.cbseonline.ernet.in

A good news for the Aspirants....
Careers360 is conducting the All India Talent search exam where students of 11th and 12th PCM/PCB can test their knowledge and enhance their preparation by knowing their stronger and weaker areas and win prizes and scholarships worth 1.25 Crore.
If you are an Engineering or medical aspirant, it is a great opportunity for you. Take the test, perform the best, avail the scholarship and win prizes.

You can win exciting prizes such as:
50,000/- cash Prizes, Upto 100% Scholarship on JEE / NEET AI-Based preparation packages, Laptops, Smartphones, Amazon Echo, Careers360 Almanacs, Powerbanks, Upto 250/-  paytm *Cashback offer

Link:
https://learn.careers360.com/careers360-talent-search-exam/?icn=qna&ici=interns

Hope this helps.

Good Luck !