All this and the comments assume Windows will let you upgrade at all. Google "windows 10 upgrade something happened" and then try to find the fix for that amazing piece of error reporting.
In my case it was either that the language pack was wrong: Eng UK not Eng US, neither of which actually have language pack installed...
or it was the Win toobar/menubar being docked to the left of the screen and not the bottom. One of these stopped the upgrade completely, repeatedly. The greatest security risk had to be getting stuck on an old version of Windows with no good info on how to fix a 2 year old bug in the upgrade process.
In my case it was either that the language pack was wrong: Eng UK not Eng US, neither of which actually have language pack installed... or it was the Win toobar/menubar being docked to the left of the screen and not the bottom. One of these stopped the upgrade completely, repeatedly. The greatest security risk had to be getting stuck on an old version of Windows with no good info on how to fix a 2 year old bug in the upgrade process.