How to modify embedded Linux firmware?

I'm sure there's a simple answer to this, however I don't usually do THIS type of programming. I purchased a device (https://amzn.com/B00INMB23Q) to copy SD cards over to a USB storage device. It currently will only transfer files once initiated by a wirelessly coected command (via web or mobile app.) My goal would be to have SD cards automatically transfer once inserted.

https://github.com/digidem/filehub-config

I found this link online with configuration files that should do what I want. I understand the code, however I have no clue how to get the files to CHANGE the device's functionality. Would really appreciate if someone could guide me with how to modify the device. Thanks!