If you have Win7 or Win8.x running, you either already have or will get an invitation for your free upgrade to Win10, appearing on your task bar. Anxious for it, I clicked 'reserve' right away, then double-thought that. I then found threads like
Win10 Offering Icon in Systray, which advise patience. It does seem reasonable to let the patches settle in before installing, so I've canceled my reservations. And I'd rather burn it in ISO form anyway. Choosing to reserve is an okay to install it right under your nose, right on top of your existing c: partition.
I don't know which is the right way to do this, perhaps no one does, being such an odd situation...