change CD key of SCCM

[sccm_freak]

i have downloaded from technet subscriber site the latest SCCM DVD with SP2 integrated already , the problem is when i do the installation it comes with a greyed out CD key and i just continue the setup without entering my own CD key that i bought from microsoft , my question is how to change the CD key to put mine instead of the one that comes integrated already with the DVD

 

thanks