When you upgrade from a genuine Windows 7 or Windows 8 installation to Windows 10, you don’t need to enter a key for Windows 10. Because you upgrade your OS with a valid key, Microsoft will automatically make your new Windows 10 copy genuine. In case you want to later on reinstall Windows 10, you can no longer use your old Windows 7/8 product key, you need a Windows 10 serial.
But where can you find that Windows 10 key? It can’t be found in the control panel (Control Panel\System and Security\System).
You will need a third party tool to show your Win 10 key. I use Speccy and it’s free to use and it even has a portable version, so you don’t need to install (and then later uninstall) a program. The latest version can be downloaded here: full version, portable version. Install or open Speccy and go to Operating System. Here, you will find the details about your current operating system. In my case:
Windows 10 Pro 64-bit
Computer type: Notebook
Installation Date: 30/07/2015 17:00:51
Serial Number: XXXXX-XXXXX-XXXXX-XXXXX-XXXXX
That’s it, your Windows 10 serial number / product key is right there. If you have logged in into a Microsoft account.