About 27-inch iMac Display Firmware Update 1.0 Updates the display firmware on 27-inch iMac systems to address issues that may cause intermittent display flickering. The 27-inch iMac Display Update will update the display firmware on your iMac. Do not disturb or shut off the power on your iMac during this update.
Source:27-inch iMac Display Firmware Update 1.0
