To clean install Windows, backup your personal files, then create a bootable Windows USB and boot your PC from that to install Windows.
Click this link:
https://www.microsoft.com/software-download/win...
to download the Media Creation Tool (Click on Download Tool Now), with that you can download the latest Windows 10 ISO (Select Create Installation Media for Another PC), you can create a bootable USB flash drive (min 8GB) using that tool
Then, Boot your PC from the Installation Media you just created (change Boot Order in your BIOS) to begin installing Windows 10
During the installation process skip the steps which ask for a product key and select the option 'I am Reinstalling Windows 10 on this PC'.