run the '/usr/sbin/netconfig' program. This a simple ncurses program. There you can change from DHCP to STATIC and also modify your IP info.
'/usr/sbin/redhat-config-network' is a flashy gui that I think does the same thing and more. If you're curious, check out the other redhat-config-* programs in /usr/sbin.
For network changes to take place, all you need to do is to 'take down ' the interface and bring it up again. If you have only one interface and its a ethernet card, then it's usually called 'eth0'. So you can do 'ifdown eth0' or the more standard 'ifconfig eth0 down', then 'ifup eth0' or 'ifconfig eth0 up'.