Hi,

If you currently have PCM and you are still in school, then you can contact your school teachers as they can guide you better to add Biology as a subject. If you are currently in 11th, then it will be easier to make the change, however it depends majorly on your school. If you have finished class 12th and need to study biology till 12th level, you can apply for 12th again along with Biology as an additional subject. If you have more doubts, you can ask questions in the comments section.

I hope this helped you.