Hello aspirant,
I am here for helping you as you are going to give improvement exam because you are not satisfied from you current marks  so, you choose a very good option and if talking about improvement exam form release date so as per the previous year record these may be in august .
If talking about pattern which will be followed in these examination so , these exams will be conducted with completely new pattern .
You can appear in any number of subjects you want but have to make sure that you can only appear in the subjects in which you have passed . Improvement exams can only be given after passing in class 12th.
The process for the same is you need to fill up the application form of the exam which can be collected from the regional offices of cbse or in the syndicate banks.Then you apply for the subjects and pay accordingly.
You can also fill up for the form online by going through the website of cbse.You have to fill in the required details and pay either online or through e-challan.
And after the successful submission of the form,you become eligible for the exam.
I hope it will be beneficial for you.................
For more queries you can upvote my answer and can ask in the comment section........
Best of luck.........