If you just want to change the NodeID, Frequency or Network Group: no need to modify the code just connect vial serial to the emonTx @115200 baud then enter [+++] then [ENTER] at startup. See video in post:
You just need to connect programmer to your computer and open up a serial window. You could use the serial monitor in Arduino IDE or there are many other serial terminals e.g. minicom on Linux. What OS are you running?
The programmer will need drivers installed on Windows / Mac, it works out of the box on Linux.