By default, batocera.linux is like a firmware. It's a file called /boot/batocera. You cannot modify it directly. However, in case of need, there are tips to do it.

# for /boot
mount -o remount,rw /boot

# for other modifications
# do any modification
# if you reboot, the modifications are lost,
# except if you saved them via
batocera-save-overlay

This script creates a file at /boot/boot/overlay file that stores all the modifications you made. WARNING: this overlay file is automatically removed at the next upgrade.

If you want to remove all your customizations, just remove this overlay file.

Note : it cannot be done for the current Odroid xu4 version.

• you generally don't need to edit /etc/fstab, use store_games_on_a_nas instead.
• you should not modify /usr/share/emulationstation/es_systems.cfg, instead, copy this file to /userdata/system/configs/emulationstation/es_systems.cfg and customize it from there. At startup, EmulationStation prefer the latter file if it exists.