That "Hello" message was most likely programmed in along with any other functionality of the radio during the manufacture of the radio circuitry and probably does not reside in something changeable like flash memory or anything. If one was skilled enough and so inclined, there might be a way to read all the data from the "ROM", edit the message, and create a new "ROM" chip to replace the current one. The cost and difficulty in doing so wouldn't be worth the bother IMO (especially for something so trivial) unless that person was very bored.