# ifconfig down ethx
# ifconfig ethx x.x.x.x netmask x.x.x.x etc
# ifconfig ethx up
edit /etc/sysconfig/network-scripts/ifcfg-ethx with the relevant information.
# /etc/init.d/network restart
ifconfig ethx down && ifconfig ethx up
Although it is better to assign a static lease with the DHCP server (on your router/modem/gateway/etc) as this will better handle conflicting IP's.