Instructions for editing the IP settings for the Gibraltar Mix Engine are in the Gibraltar Network Manual, Appendix 1. Here's the link to the manual:
http://wheatstone.com/support1/download-all-literature-manuals-software/0-gibraltar-and-bridge-tdm-networks/gibraltar-bridge-tech-manuals/30-gibraltar-network-manual

There are also instructions for editing the IP addresses of earlier control surfaces, but the procedure is different for the Dimension Three: you must connect to each CPU using a web browser which will bring up a web form to change the IP settings. The credentials to log into the web interface are:
User = knockknock
PW = whosthere

When you change the IP address of a Gibraltar Cage or control surface with a backup CPU, be sure to make the settings identical for the Primary and Backup CPU (with the exception of the name - the software doesn't care about the name). Each CPU will use the appropriate IP address based on its status. It will use the Primary IP address if it is actively controlling the hardware, and will use the Secondary IP address if it is in the backup role. Example:

If the Primary CPU has the following settings...
NAME:CPU A
TIERID:1
NUMTIERS:6
IPADDR1:192.168.1.160
GATEWAY:255.255.255.255
SUBNET:255.255.255.0
BCAST:55555
FAILIP:192.168.1.161

The secondary CPU must have the following settings (The name doesn't matter):
NAME:CPU B
TIERID:1
NUMTIERS:6
IPADDR1:192.168.1.160
GATEWAY:255.255.255.255
SUBNET:255.255.255.0
BCAST:55555
FAILIP:192.168.1.161

Notice that the settings are identical on both, with the exception of the name.