Via command line to change the Product key:
- Click on the Start Button and type: CMD (into the ‘Search programs and files’ field in the Start Menu).
- The Start Menu will show ‘CMD.exe’,
- Right-click the CMD.exe file and select Run As Administrator .
- In the CMD window, type: slmgr.vbs -ipk <type your product key here.>
- To activate windows after changing the key, type: slmgr.vbs -ato