Here are some rough notes describing how to update the BIOS on a Lenovo T420s laptop. Lenovo offers a BIOS update utility that runs under Windows or as a bootable CD. Both of these options did not work for me as I was running Linux and had no blank media for burning a CD.
Instead I plugged in a spare flash drive and did the following:
Download the latest ‘BIOS Update Bootable CD’ from Lenovo.
You can’t directly copy this ISO to a flash drive since the ISO is formatted as a bootable CD. Instead, extract the boot image using a Perl script called geteltorito.pl.
wget -O geteltorito.pl https://github.com/ksergey/thinkpad/blob/master/geteltorito.pl
Run geteltorito.pl with the ISO image as first parameter and redirect output to a file.
perl geteltorito.pl xxxxxxxx.iso > bios.img
Use dd to copy the newly created file to a flash drive (where sdX is actual flash drive letter).
sudo dd if=bios.img of=/dev/sdX bs=512K
Reboot. Boot from flash drive and update BIOS.
Make sure you download the correct BIOS version. I initially downloaded the BIOS for the T420 instead of the T420s.