From Linux on Power wiki
Sometimes, bugs or problems may be corrected only by updating the firmware (microcode) running on the system. This page breifly reviews some of the firmware update options. A complete set of documentation relating to mcrocode update is available at the Microcode update technical information page.
Firmware may be obtained from the i5, OpenPower, p5, pSeries, and RS/6000 system microcode download page.
Two copies of the firmware
In order to minimize damage that might be caused by a failed firmware update, the system actually contains two copies of the firmware. One copy is called he "permanent side" and one is called the "temporary side". Either copy may be booted, so that if one doesn't boot or doesn't work as expected, the other image should still be usable.
During firmware installation, the currently running firmware is marked as "permanant", and the newly installed firmware becomes the "temporary side".
Firmware may be updated either from the HMC, or from the Linux command line.
Updating the firmware from the HMC
The firmware may be updated from the HMC GUI or from the HMC command-line. The firmware may be installed either from a CD-ROM, or by specifying a download site to te HMC installer.
- Need more info here.
Updating firmware from the Linux command line
After downloading the firmware from the above URL, one also needs to obtain the "update_flash" utility to actually install the firmware. The "update_flash" command is a part of the "ppc64-utils" package, which may be obtained from Linux on POWER service tools site.
For obvious security reasons, the update of firmware from a Linux partition is normally disabled. In order to flash, it must be enabled on the HMC as described on the firmware update control page. For non-HMC connected systems, firmware flash mst be enabled through the menus on the FSP.