If you want to update via USB, there is this:

USB Booting Guide

Latest MorphOS updates also include a so called hybrid hdimg which also can be used to boot from an USB stick but this requires both a compatible USB stick and obviously will overwrite all your previous data on that stick as you basically write a raw image to the stick on block level.
I use a dedicated 2GB USB stick for that but those are rare to find these days. It also requires to change the boot device in OpenFirmware to USB as Apple's OpenFirmware doesn't find it automatically.