Hi everyone!
Today I updated my lappy to Win 10 but found some scrap from old windows 8.1 so I called Microsoft and asked them about a link to the Win 10 iso file for clean install! And they mailed me with it))) So I asked what will be with my activation and they told me that as I updated firs from win 8.1 to win 10 my serial number was linked to my hardware and now I can just simply burn an iso on disk or flash drive and do a clean install.

So here is my small instruction:

1. Download an app from Microsoft's link which they sent you.
2. Follow simple instruction.
3. Burn an iso file on a disk or flash drive
4. Install your win10 (in my caseit was win10 home x64 rus)
5. Skip all asking to enter a key))) (in second window there is a small sign in the lower left corner it says: ask me later)
6. Enjoy!!!

Don't forget that clean install will remove all your files from system partition. And everything you are doing at your own risk! I'm just sharing my experience but in your case something may differ!