So of course Microsoft being Microsoft they are very vague about licensing which makes the whole thing somewhat unintuitive.
This is what i figured out myself, and it works for doing a complete fresh install of Windows 10 and activating it if you are currently running Windows 7 or 8(.1). 1:
First things first, make sure you have everything backed up and saved. Also write down your current key for Windows 7 or 8.1. 2: Then download this little tool
https://www.microsoft.com/en-us/software-download/windows10 .
And use it to download an ISO for your version of Windows. Got Windows 8.1 Pro? Go with Windows 10 Pro. Got Windows 7 Home Premium? Go with Windows 10 home, etc..
3: Make a bootable USB stick with this:
https://rufus.akeo.ie/ . Use GPT for partition table, and file system FAT32.
4: Then put the USB stick aside and install the Windows 10 upgrade through Windows update. The reason being that your old key won't work to activate Windows 10. But during the update process you automatically get a new key, a legitimate Windows 10 key.
5: Now once it's all installed use this tool:
http://www.nirsoft.net/utils/product_cd_key_viewer.html to find your new Windows 10 key. Make sure to write this down, so you can always find it.
6: Now with your new Windows 10 key you can install Windows 10 and activate it like you're used to. Boot from the USB stick, install Windows 10, install the drivers, activate and off you go. I hope this is helpful for someone out there, because it took me a while to figure out as well since there are 0 tutorials explaining this right now.