[arm-allstar] RPI3 Halt Button

[Chris Souleles]

Excuse if this is a duplicate - I may have previously posted it to the wrong list.  
I'm new to Allstar and I don't know linux.  I want to add a halt button to my RPI3 node.  I see directions on how to add the button which is straight forward and then it says to go to /usr/local/etc/allstar.env and set SHUTDOWN_MONITOR="enabled.   My question is how do I find /usr/local/etc/allstar.env to make that mod.  
Thanks, Chris - W6CS,  73