Hello.
If you have paid the fee that means your application is successfully submitted now it is quite difficult to change the subject but NIOS has mentioned that you can change by writing an application on a plain paper and submit it to the regional office of NIOS with the free prescribed by them you can see the fee on the official website which is mentioned below, after submitting the application they will be in touch with  you then further process will be explained.
https://nios.ac.in/student-information-section/procedure-for-change-of-subjectadditional-subject.aspx
Best of luck.