The free Windows 10 upgrade offer expires after July 29th, 2016.
How to directly clean install Windows 10 without having to upgrade first.
If you have an OEM computer with UEFI firmware (BIOS) embedded Windows 8/8.1
product key, the Windows 10
installer should automatically detect product key from UEFI firmware (BIOS) and you will be taken straight to the License Agreement screen. When installation is finished, Windows 10 will be activated automatically, if your internet connection is up and running.
But if you are being prompted for a product key, it means that you have the wrong installation media and that's when the Windows 10 installer can't detect Windows 8/8.1 product key from UEFI firmware (BIOS). Note:
Do not select the "I don't have a product key" option, because activation is not successful. You need to download the correct installation media of Windows 10.
How to create a bootable Windows 10
USB flash drive or DVD. -> link