Hello

Your question is not specific about where you want change in subjects or in entrance paper.

Let's talk about both yes you can change your PCB to PCM subject .if you are not interested in biology then you can change it.

For your entrance exam if you have maths subject then take PCM or if you have bio subject then take PCB. If you have all subjects then think about your interest and select .

But after you fill form for entrance you are not able change subject.

I hope this helps you