If you've got a a valid Windows 7 Ultimate, Windows 7 Professional, Windows 8 Pro or Windows 8 Pro for students product key and you're running Windows 10 Home, why? You can use a valid product key for any of the previously mentioned Windows Editions to upgrade to Windows 10 Pro!
But, can I still do this after the free upgrade period?
The answer is probably, for more information, please read this post.
Well, how do I do it?
Before you do so, please note that this may not work/may stop working as Microsoft have stated the the free upgrade offer for Windows 10 ended July 29 2016. For more info, read this post.
Open up your Settings app and head over to System & Security and then Activation. After that, click/tap the Change product key link and then, to initiate the upgrade, enter the default Windows 10 Pro product key (VK7JG-NPHTM-C97JM-9MPGT-3V66T) when prompted to do so, if all goes well, Windows will ask if you want to upgrade to Windows 10 Pro, after confirming that you would like to upgrade, Windows will continue with the upgrade. After it's done, repeat the process above but instead, enter a valid product key for one of the editions listed above, Windows should then activate.