As per the instructions from HP for windows systems:
If the device is shared on the network, then the following command can be issued from a command prompt:
copy /b FILENAME \\SHARENAME\PRINTERNAME
where FILENAME is the name of the .RFU file,SHARENAME is the name of
the machine, from which the printer is being shared, and PRINTERNAME is
the printer's share name.
If the device is attached locally, then the .RFU file can be sent
directly to the printer with a COPY command from a command prompt or
copy /b FILENAME PORTNAME
where FILENAME is the name of the .RFU file and PORTNAME is the name of the appropriate device port (such as LPT1).
The following can be set on the computer to help address a timeout message that may occur during an update:
From a command prompt or DOS window type the following command and press the enter key:
If the device is attached to a parallel port other than LPT1, then
substitute the proper LPT port number in the MODE command. Once this
command has been set the file can then be copied to the printer with
the above mentioned COPY command.
When the upgrade process has completed the LaserJet will
reboot. Once the printer is back online, print a Configuration Page and
verify that the new firmware has been installed.