Can someone direct me to information on how to program or modify via Arduino or any other system to customize the RTK unit?
Thanks in advance.
https://docs.sparkfun.com/SparkFun_RTK_Firmware/intro/ is a good starting point 
I have read the document referenced. However, I get an error message in the Example1_BasicNMEARead.ino example. Can you please tell me how to connect the data cable provided with the Sparkfun RTK Express to an Arduino? I assume the GND black wire goes to the GND on the Arduino. Where do I plug the TX and RX wires?