Things have changed, really changed, when it comes to upgrading Editions under Windows 10, and it's much easier than in the days of old.
You can upgrade from Home to Pro at any time if you have a Windows 10 Pro key. Every Windows 10 installation has all of the components needed for every Edition (Home, Pro, Educational, Enterprise) and it is the license key that determines which are actually activated and, thus, which Edition you run. For the sake of example, I'm going to presume the machine in question is currently running Home, but the method applies regardless of the Edition you're currently on, if you're "movin' on up!"
Open Settings, Update & Security, Activation Pane, then activate the Change product key link under the Upgrade your edition of Windows section. After you activate it you will be led through the steps to enter your Pro product key and have the existing system "go Pro."
Once Windows finishes the voodoo it needs to do under the hood, voilà, you are now running Pro!
Edited by britechguy, 14 February 2019 - 11:31 PM.
Eliminated a step by going directly through Update & Security, Activation Pane