I had to figure this out and decided to share to hopefully save someone a hour or so.
To update the BIOS on an older Lenovo desktop / laptop, which I think are denoted by “Bootable CD” on Lenovo’s site, require converting the image to a new format and disabling SecureBoot.
Newer UEFI/BIOS updates are UEFI-signed images and denoted with “CD ISO image version” and don’t need to be converted, you can just copy (with dd) the image to a USB and flash.
Convert the “Bootable CD” to an image format using:
geteltorito -o uefi.img lenovo_bios.iso
Then flash it to a USB drive
sudo dd if=uefi.img of=/dev/sdz where z is your usb device (check in dmesg once you plug in your device)
Then go into the BIOS/UEFI and disable Secure Boot, reboot and press F8 / F12 and select the USB.
That’s all!