To modify ip-tables directly in ubuntu
First save off your current config:
$ sudo -s
$ iptables-save > /root/working.iptables.rules
$ iptables-save > /root/working.iptables.rules.to.modify
At this point you can modify the file. Once your are satisfied restore it
# iptables-restore < /root/working.iptables.rules.to.modify