The OpenELEC distribution uses squashfs  which is read-only. If you installed OpenELEC with Berryboot, you have to had folders named
/images on your SD card or USB stick. So if you want to edit a file which changed by OpenELEC you can find it in
/data folder. Put your SD card into another computer, find the data folder, and edit it.
To change boot files, such as config.txt, you have to mount your SD card in another computer with
mount /dev/mmcblk0p1 /mnt command, then go into /mnt folder and edit your file.
By the way, /images folder contains your Linux distribution images which are downloaded by Berryboot.
The second way to edit OpenELEC files is to uncompress the OpenELEC image file with the unsquashfs tool. First install squashfs-tools,
apt-get install squashfs-tools
Edit your files then:
mksquashfs /openelec OpenELEC-modified.img
And put the modified image into the /images folder in Berryboot.