Backup your files first.
You will need an empty 8GB USB flash drive for the installation USB, because the process to create the bootable USB will delete all content on the USB.
Click this link:
https://www.microsoft.com/en-us/software-downlo...
to download the Media Creation Tool (Click on Download Tool Now), with that you can create a bootable USB flash drive (min 8GB) using that tool (Select Create Installation Media for Another PC),
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'.