How to get a new login ID with switching command from Mac to windows.

Hello,

I've got a gift from my Boss iMac 21.5", and, its Serial No. DG*******G7F.

It might be used just for couple of times, and only window part he used as well as me too.

Today it has been switched to Mac and asked to my boss for the pass key.

My boss can't recall the pass key, and, Im not abling to switch to windows part from the mac to pressing the option key.

Would you please help me to advising accordingly.

Kind regards,

Anupam