Change permission on file in /sys folder
Yes, your phone must be rooted because to access that file the /system needs to be mounted as write-able

Once you obtain root you can run this command inside a shell:
su
chmod 777 /sys/class/leds/button-backlight/max_brightness

Then just navigate to that file and edit it using a file browser. What the chmod 777 command does is change the permissions on that file so that you can edit it. After you finish I would suggest changing the permissions back to what they were.

The original permissions were probably 0666.
So it would be like this: chmod 0666 /sys/class/leds/button-backlight/max_brightness

To encompass t0mm13b's remark below, go here for information on how to make the file write-able at boot, instead of having a script be run at each boot that makes it write-able.