Other Mint users have indicated that Xed is the text editor that is standard for Mint.
Try using it and making edits to the configuration file.
With the built in text editor in Mint I tried to go to directory where Auto-CPUFREQ program is and I don’t see configuration file perse, I just see the configuration preview. Is that the same thing? I did try to change the variables within that text file and saved it, then rebooted Mint, and once I was plugged into AC power boost was still ON and “performance” was still selected instead of powersave. Not sure if I am doing everything correct though as I’m not very fluent on Linux.
I saw the developers notes saying to access the config file it is at /etc/auto-cpufreq.config. I'm not even sure how to find that directory using the typical "start menu" button on bottom left of desktop.