(not USB flash drive, unless you want a bootable USB).
While Microsoft no longer hosts a direct, public "Download Windows 8.1" landing page for standard retail users, there are still official and community-verified ways to obtain the image: download windows 8.1 iso 64 bit
If you need to install or reinstall an older operating system for legacy hardware or software testing, you can still file. However, because Microsoft officially ended support for Windows 8.1 on January 10, 2023 , the public download options have changed. Official and Verified Sources (not USB flash drive, unless you want a bootable USB)
An ISO file is not usable on its own; you cannot just copy it to a USB drive and expect it to install. You must make the USB drive "bootable." (not USB flash drive