apt-get install squashfs-tools
mount -o loop livecd.iso /mountpoint/iso
unsquashfs /mountpoint/iso/casper/filesystem.squashfs
# make your changes in squashfs-root
mksquashfs squashfs-root filesystem.squashfs -b 1024k -comp xz -Xbcj x86 -e boot
# cd inside the extracted ISO content and create the isofile somewhere else
mkisofs -J -l -R -v -o ../livecd_new.iso ./