Firmware updates require that you press the Interrupt button, identified in Figure 1, while turning the computer on. Use a thin, blunt object such as a straightened paper clip or the tip of a pen.
Note: The Interrupt button on older iMacs is recessed so you will need to use a paper clip.
The Interrupt button is located on the front panel of the Power Macintosh G3 and Power Mac G4 computers, and on the right side of iMac computers. Refer to your Users Guide or Mac Help for the location of the Interrupt button on other computers.
Figure 1. Programmer's or Interrupt button
For specific firmware update installation instructions, please refer to the Read Me file that comes with the update.