Default gateway changing

To temporarily change the default route you can use an ip command like this:

$ sudo ip route change default via 192.168.1.1 dev eth0

If you have a static ip configuration in /etc/network/interfaces you can add a gateway statement to make this permanent.

iface eth0 inet static
    gateway 192.168.1.1
    […]

Using DHCP to configure networking you have to adjust another file instead. Add the supersede statement in /etc/dhcp/dhclient.conf.

supersede routers 192.168.1.1;

Open the file /etc/network/interfaces

find desired interface and add following:

gateway 192.x.x.x

Restart network:

sudo /etc/init.d/networking restart