Yes, it’s a file on your gateway’s Pi, so using SSH, and then use an command line editor such as nano /opt/ttn-gateway/bin/local_conf.json, might be easiest. What command do you use to start the gateway? That should be in /opt/ttn-gateway too.
You might need to be root to edit the file: sudo nano /opt/ttn-gateway/bin/local_conf.json.
By the way, to create a random EUI using @terrillmoore’s little tool:
git clone https://github.com/things-nyc/random-eui64.git
cd random-eui64/
make random-eui64
I ignored a warning on a Mac, warning: format specifies type 'size_t' (aka 'unsigned long') but the argument has type 'unsigned int'. Next, run:
./random-eui64