How To
Open Settings > System > About > Change Product Key or Upgrade Your Edition Of Windows > Change Product Key
Now you’ll want to enter the default Windows 10 Pro product key as mentioned by Charles From Microsoft:
From your Windows 10 Home running Version 1511, enter the Windows 10 Pro Default key under change product key.
VK7JG-NPHTM-C97JM-9MPGT-3V66T This default key will not activate the system, just take you to Pro so you can activate using a valid Pro key that you will provide.
Once you have done this, the system will go through an upgrade process from Home to Pro, but will not be activated on Pro. You can now enter your Windows 8 Pro key into your Windows 10 Pro system and it will activate.
After you perform these steps, your computer will restart a few times and upgrade your edition to Windows 10 Pro. This new upgrade will not be activated, so you will want to follow these steps again and enter your legit key for Windows 8 Pro or Windows 10 Pro.
Open Settings > System > About > Change Product Key or Upgrade Your Edition Of Windows > Change Product Key