Hi Blooxxel and welcome to the Magix User to user forums.
You have to go to your registered email account and log in to your personal space where you first registered the program using the same email address you used then. There you will be able to de-register your product.
[Edit]
I just re-read your enquiry. You can't. it's impossible. Only the registered owner of the copy can do that. Sorry.
Even then it's doubtful you could re-register it as you would need the owner to replace their email address with yours along with a password for access. You would then have to log in and change the password before going further.