Windows 10 Download Official ISO

To underline some aspects. To use the official ISO images of Windows 10 you must have a valid Product Key (Retail / OEM). Therefore, before you can use the ISO for a clean installation, users must upgrade their PCs so that their original Product key (Windows 7 or Windows 8/8.1) can be recognized by Microsoft as a valid Product Key for Windows 10. Upgraded, quote from that moment you can restore your PC using the official ISO and license already in your possession.

http://go.microsoft.com/fwlink/?LinkId=616935 (32 bit)
http://go.microsoft.com/fwlink/?LinkId=616936 (64 bit)

More detail :
https://www.microsoft.com/en-us/software-download/windows10