Hi,
Find the Windows 10 media creation tool on the Software download page
"Start fresh with a clean installation of Windows 10"
https://www.microsoft.com/en-us/software-download/windows10startfresh
"How to Clean Install Windows 10"
http://www.tenforums.com/tutorials/1950-windows-10-clean-install.html
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When your free upgrade to 10 was originally activated Microsoft will have kept the Hardware Configuration details of your device on their Servers for recognition purposes if you have to reinstall 10.
"Activation in Windows 10" (Digital entitlement)
https://support.microsoft.com/en-au/help/12440/windows-10-activation
Cheers.