If you have an old DVD or USB from when Windows 8.1 was current, that ISO is authentic. You can rip it to ISO using ImgBurn or AnyBurn.
Looking for a Windows 8.1 ISO? Softlay is a popular third-party source. This 2000+ word guide covers download steps, installation tips, security risks, and official Microsoft alternatives. windows 8.1 iso softlay
Developers with active MSDN subscriptions can download official, unmodified Windows 8.1 ISOs directly from Microsoft. If you have an old DVD or USB from when Windows 8
Boot from USB: Restart your PC and enter the BIOS/UEFI settings (usually by pressing F2, F12, or Del). Change the boot order to prioritize the USB drive. windows 8.1 iso softlay