You don't, but you can only flash the BIOS from Linux.
sometimes, I've installed and run Windows to update a firmware or bios or unlock a phone just because it could be done from another OS but its too much hassle working out how to bother for a one-time task which is already documented. It looks like in this case, there is a documented method using Linux, so.. thats simplest.
You can flash the BIOS on the APU from OpenBSD. Just `pkg_add flashrom`.
Sweet! Hopefully the author sees this: his site mentions using Linux for flashing.