T420 BIOS update fails with "BIOS directory not found" error

[FastCabbage]

So, I must admit to not being very tech savy, and this may well be an obvious mistake on my part. I have been blindly following online instructions without any real understanding of what they entail.

I am currently trying to upgrade my dad's old T420 running Windows 10, but before swapping out hardware I am trying to update the BIOS (from 1.26 to 1.52), as I saw one person had an issue with outdated BIOS preventing startup after upgrading his CPU (to i7 2760QM, same as me). I am attempting to update BIOS from a DVD, to which I burned the 83uj33us.iso I downloaded from lenovo using ImgBurn. On the boot menu I'm kinda going by guesswork, hitting F12 to "choose a temporary startup device", selecting the DVD, then "update system program". At his point I get an error reading "BIOS Directory was not found. The process did not complete".

I assume I have either made an error in how the iso was bured, or through my guesswork in the boot menu, but I'm not sure where to go from here. I may dig around for a USB drive and try the Rufus route.

Any help would be greatly appreciated, either in explaining the error of my ways, or perhaps pointing out that this is a fools errand and I can leave the BIOS be before ungrading hardware. Thanks in advance for any advice