Instructions are on Page 159 of the hardware maintenance manual.
Upon installing the BIOS version 73 from Windows (using silent install switches), the PC rebooted and started beeping the CMOS checksum error beep (two short beeps). Clearing the CMOS with the jumper didn’t help.
To make a long story short, burn the BIOS update to disc or copy to a USB flash drive (download the appropriate version from the Lenovo site). Then move the BIOS reset/recovery jumper (bottom right part of the board) to the right, put in the CD or USB, and boot the system.
First, I tried burning the v74 ISO to a DVD, but the recovery process never started. Instead, I formatted a USB drive to FAT (1024 sectors, not sure if that made any difference) and extracted the ZIPped BIOS update to the drive. I inserted it into the PC, powered it on, and the normal BIOS update screen came up.
The system will power off when it’s done, then you need to move the jumper back to the original position before powering on the system.
The moral of the story: use the same version to recover that you were trying to originally flash. Using v74 to recover after trying to flash v73 didn’t work, but recovering with v73 did. The site says to use a CD/DVD, but a USB drive also works.