Since you no longer have the Media Creation Tool to automatically format your USB drive, you must use a third-party tool like Rufus to turn the ISO file into a bootable installer.
: Choose specific languages, architectures (32-bit or 64-bit), and editions (8.1 Standard, Pro, or Single Language). media creation tool windows 8.1 download
If you are considering reinstalling Windows 8.1 to revive an old PC, think about these safer options instead: Since you no longer have the Media Creation