The process of changing your serial key isn’t like it was on Windows 7 – it won’t give you an option to modify it through the activation screen. Do the following to open the menu to change your key:
1) Open the Windows + R button on your keyboard.
2) Type “slui 0x3” without the quotation marks in the window titled “Run” and press enter.
From here you will have the option to enter your key.