Depends on the firmware version already installed: to check this, press and hold the "Display" button on the unit, then press "Standby".
Very early firmware versions require a hardware modification before they can be updated; later versions can be updated using an RS232 connection or using a CD player with an optical digital output.
Best bet is to make a note of the firmware number, and have a word with Onkyo's official UK service agent, GenServe, who are extremely helpful.
My 875 required the hardware mod before it could be updated, and so I took it down to GenServe, who did the job within a couple of hours.