Method 1: Command Line Utility
You can use a great command line tool that will help you do this very quickly. Just follow these steps:
Launch the Start Menu and type in command prompt or cmd so that it shows up on your start menu search list. Right click on the Command Prompt shortcut and select Run As Administrator.
At the administrator command prompt, type in “slmgr.vbs -ipk ”
slmgr.vbs -ipk 00000-00000-00000-00000-00000
To activate windows after changing the key, run “slmgr.vbs -ato”
Method 2: Using the Activation UI
Alternatively, if you want to use a graphical interface, run the following at a command prompt as suggested by Dave in the comments.
Then just type in your product key and click the Activate button.