If you bought a computer with preinstalled Windows 10 Home and would like to upgrade to Pro, you have to buy Windows 10 Pro in order to get the Pro product key. In my case buying a full OEM version was cheaper than upgrade. But even having got the product key for the Pro version, you cannot simply upgrade by changing your product key. Even clean install would not help. In the BIOS of your computer there is an OEM Marker. So if the computer originally came with preinstalled Windows 10 Home, exactly that version would be installed even if you perform clean install from the install media of the Pro version. You won’t be even asked for a product key as the one in BIOS would be automatically taken.
So you need to use the following workaround. Upgrade to Pro using the default product key:
Then change the procuct key to your Pro’s one.