Basically as a reminder for myself here are the steps to update the BIOS on a Supermicro X9SRiF board. The reason in my case was that I needed bitfurcation and that was not available in the v3.2 BIOS installed before.

• Download ZIP file with new BIOS (v3.3 is the latest as of this writing and will likely stay, given how old the board is). The release notes are available here.
• Use Rufus to create a USB stick that boots FreeDOS.
• Extract the contents of the BIOS update ZIP file onto the USB stick.
• Boot your X9SRi-F board off of the USB stick by pressing F11 to enter the boot menu at startup.
• Run the update program with the command ami x9sri8.529
• Wait patiently until the command prompt comes back.
• Turn the machine off, wait at least 10 seconds, and turn it on again.
• It will switch itself off once, during the first startup, so don’t get nervous.