If you no longer have your key but Windows 8.1 came preinstalled on a device, the key may be embedded in the BIOS/UEFI, and installation might activate automatically with the correct edition.
This trick often bypasses the OEM key restriction and allows the ISO download. download windows 8.1 disc image -iso file- - microsoft
Get the official SHA-1 checksum from Microsoft. Unfortunately, Microsoft no longer publishes these publicly for older OS versions. As a reliable alternative, use the following known good hashes for the final Windows 8.1 with Update (x64) — English: If you no longer have your key but Windows 8
