Yes friend you can definitely give improvement exam on one subject or on any number of subjects which you wish.

As far as I know June is the last date to register for the improvement exam.

You will have to give a written application to the Principal of the School, who will register you online for the entry to the ISC Examination.

If you want more details you can send e-mail to the council. The email id is council@cisce.org They help you real quick.

Good luck :)