Hi Ranjan,

If you are in Class 12th with PCM and now you are willing to choose biology i.e. PCB, then you can switch to it but you will have to start again from 11th class with Physics, Chemistry and Biology. But, that will result in wastage of two years of your life, and this is not at all OK, you should should have thought it before.

Feel free to ask doubts in the Comment Section.

I hope this information helps you.

Good Luck!