Search /etc/named.conf to find the zone file for the domain.

Find the domain name and see where the zone file is.  Example zone block.

zone "incredigeek.com" in {
 type master;
 file "/var/named/mzones/incredigeek.com.hosts";
 allow-query { any; };
 forwarders {};
};

The file is /var/named/mzones/incredigeek.com.hosts

Edit your zone file by opening it up in a text editor.

Example.  Text in bold added for comments.

$TTL 21600
$ORIGIN com.
incredigeek IN SOA dns1.dns-server.com. dns2.dns-server.com.(
 0000147 ; serial  <- This needs to be incremented so it is greater than the previous version of this file
 43200 ; refresh (12 hours)
 7200 ; retry (2 hours)
 604800 ; expire (7 days)
 21600 ) ; minimum
 NS dns1.dns-server.com.
 NS dns2.dns-server.com.
 300 A 10.0.0.11   <- A record for root domain
$ORIGIN incredigeek.com.
localhost IN A 127.0.0.1
www 300 IN A 10.0.0.11  <- www subdomain A record
login 300 IN A 10.0.0.12 <- another subdomain A record

Save file and reload Bind

On FreeBSD

rndc reload incredigeek.com

you can reload everything with

rndc reload

On Fedora/CentOS/REHL

service named reload

On Ubuntu/Debian

service bind9 restart

You may need to reload Bind on any slave servers