The best and cleanest solution is to run "sys-unconfig". This will clear out a number of settings such as network settings, and then reboot the system.
On reboot you will be prompted to enter the new network settings, including IP address/netmask/DNS servers/etc. Alternatively it will give you the option to use DHCP is that's what you would prefer.
It is also possible to change the IP by manually editing a number of files on /etc, but I wouldn't recommend it - sys-unconfig is much easier and is guaranteed to update the right files for you.
