Hi

I am extremely sorry to tell you dear but you cannot change any subject  in your class 12 even if it's an additional subject.

This is because,

As per the rules of CBSE , a student is not permitted to change his/her subject of study after passing Class XI. He or she will not be offered any subject in Class XII which she or he has not studied and passed in Class XI.

So, you cannot change your additional subject from Maths to Agriculture in your class 12

Thankyou