To add to what patcooke said, there is unfortunately no legitimate direct links on the web as there is for Windows Vista/Windows 7, so running the Upgrade Assistant from a 64-bit PC is your best bet. It will offer to create and burn an ISO to a disc or it will let you create a bootable flash drive to install from (so long as the flash drive is 4 GB or more). Peronsally, I always chose the latter as it makes the installation go so much faster (read speeds are higher from a USB drive than a DVD).