Microsoft does not allow you to download Windows 10 directly. If you try it from a Windows 7 system, it will ask you to upgrade it. If you have MSDN subscription you can directly download it.
There is a small trick to download it from Firefox on a Windows 7 machine and without any MSDN subscription. This is a genuine download from Microsoft and not pirated or cracked version.
Here it is. Thanks to techdows.com:-
If you’re in Windows 7 or 8.1
1. Right click on the Firefox browser shortcut.
2. Select Properties > Compatibility > ‘Run this program in compatibility mode for’, and choose ‘Windows XP (Service Pack 3)’ from the drop down and Click ‘OK’.
3. Visit this page in the browser, it now opens without redirecting.
4. Select Windows edition, in general, you should be choosing ‘Windows 10’, click on ‘Confirm’ button after that, next select the language as ‘English’ and confirm again.
5. Now 32-bit version and 64-bit version download links appear for Windows 10, begin the download, it will be picked up by third-party download manager like IDM if you’ve integrated into the browser.
Thanks to the below link for the details: