We know that Microsoft did not give offline ISO files of Windows 8.1 for public download. If you use Windows 8 and need to upgrade to Windows 8.1, you'll have to download and install Windows 8.1 by using  Windows Store.

Some folks need to perform a clean installation of Windows 8.1 operating system in their computers, however they can not as a result of there's no official place to download Windows 8.1 setup ISO files. therefore 1st they're going to got to install Windows 8 and so upgrade it to Windows 8.1 which might not be a clean installation of Windows 8.1 .

Microsoft has a free official tool to download Windows 8.1 setup ISO and build its installation media. you'll use this tool to store Windows 8.1 ISO file to your pc and later you can  produce bootable USB flash drive or bootable dvd to put in Windows 8.1 in any pc.

You can download and run this tool in any Windows version however you will need internet connection because it downloads full ISO file of Windows 8.1 OS.

It allows you to pick out your required language, Windows 8.1 edition and system design (32-bit or 64-bit).

Once you decide on the desired details, it asks you whether or not you would like to make a bootable USB flash drive of Windows 8.1 or need to save the setup ISO file of Windows 8.1 for later use.

You should choose the 2nd choice to save ISO file so once downloading and saving the ISO file, you'll produce a bootable flash drive or burn it to a USB drive or make DVD by using  Windows or any third party disc burning utility.

Download Windows 8.1 Installation Media Creation Tool