You should be able to edit /etc/defaultrouter - it should have rw-r--r-- on it as permissions.

The following copied from SUNSolve Doc ID 21252 free info doc on changing IP dated 19 Dec 2000 -

Normally to change the hostname for a machine the sys-unconfig command is used.
sys-unconfig affects the six files detailed below.

Note #1: This procedure should not be used for NIS+ masters/replicas or NIS masters.
This is only for client machines.

Note #2: This only changes the information locally. If the system is running NIS/NIS+
and/or DNS, the changes need to be made in these databases as well.


To change the hostname without having running sys-unconfig, edit the following
files:

1) /etc/hosts (change file entry to the new hostname)

2) /etc/nodename (change file entry to the new hostname)

3) /etc/hostname.<interface> (where <interface> is the name of the primary
interface for this system, i.e hostname.hme0 or hostname.le0. Change file entry to the new hostname.)

4) /etc/net/ticlts/hosts (change both columns to the new hostname)

5) /etc/net/ticots/hosts (change both coluums to the new hostname)

6) /etc/net/ticotsord/hosts (change both columns to the new hostname)

To change the IP address for a machine without running sys-unconfig, edit the following files:

1) /etc/hosts (change the IP address)

2) /etc/netmasks (if subnetting is used)

3) /etc/defaultrouter (to specify the new gateway for this subnet)


As always, check the man pages as your system may (and seems to be) different from the norm.