Hi

I am extremely sorry to tell you dear but now you cannot change your subjects from PCB to PCM.

This is because,

You have already passed your 12th board examination with PCB , so nothing can be done. The only option left is to study your 10+2,  again and choose PCM this time.

One  more option is to directly give 12th board examination from recognised open boards like NIOS with whatever subjects you want i.e PCM in your case.

I hope that you find this helpful

Thankyou