If you’ve got access to a Windows machine, the official method for downloading ISOs for Windows 8.1 and 10 is the Media Creation Tool. Download the Windows 10 or 8.1 ISO Using the Media Creation Tool The most important thing is that you have a valid license key. If it won’t activate, you can always install and then call Microsoft to get them to straighten it out and allow your copy to activate. NOTE: Depending on the OEM version of Windows that you are running, you might run into an issue using the OEM key with a retail version of Windows. Instead, you get official installation media straight from Microsoft.
There are a few ways to do this, but they’re all on the straight and narrow–you won’t have to visit a shady BitTorrent site to download ISOs that may be filled with malware.