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Dependencies: bsdtar(app-arch/libarchive), mkisofs(app-cdr/cdrtools), sys-fs/squashfs-tools
In this case we open and edit a Debian based LiveCD
Create directory for extract iso

   mkdir $HOME/livecd

Extract ISO content to created directory with bsdtar

  bsdtar -C $HOME/livecd -xf LiveCD.iso

Find the place where the filesystem squashfs is in this chace $HOME/livecd/live/filesystem.squashfs and extract it.

  unsquashfs live/filesystem.squashfs

Now apply changes to extracted directory (./squashfs-root)
Create a new squashfs which contain changes

  mksquashfs squashfs-root/ filesystem.squashfs

Finally create a new patched image of the LiveCD

  mkisofs -o LiveCD_patched.iso -r -J -no-emul-boot -boot-load-size 4 -boot-info-table -b isolinux/isolinux.bin -c isolinux/boot.cat $HOME/livecd