In general, you copy the ROM image to flash, much like you would an IOS image.
Then you often use a "special" command to update the ROM flash. On some platforms, when this completes, you need to reboot immediately for the new ROM to be used. (I.e. on such platforms, if you "defer" the reboot, the next reboot will continue to use the existing ROMMON image.
Also, generally, there's documentation, on how to do the foregoing, on Cisco's main site (usually release notes for your platform's ROMMONs) that have all the details, including any special upgrade and/or prereq requirements.
