You still need to compile it, even if you edit the source file in situ, and then upload it to the Atmel '328P
My crib notes have a link to the full procedure to edit the emonPi sketch & reload:
Power factor on Emoncms from emonPi - #2 by Robert.Wall
That was for somebody who wanted to extract Power Factor - that’s close enough to what you want to do for you to be able to follow.
There have been recent changes to emonCMS, so I’m not certain that this is still totally accurate - that depends somewhat on how up-to-date your emonCMS is.