How to modify iptables rules via editing a file rather than interacting via command line?

In CentOS you have the file /etc/sysconfig/iptables if you don't have it there, you can create it simply by using iptables-save to dump the current rule set into a file.

iptables-save > /etc/sysconfig/iptables

To load the file you don't need to restart the machine, you can use iptables-restore

iptables-restore < /etc/sysconfig/iptables