How to prevent file uploads to the Internet from within the LAN

You need to configure squid.conf to enable the transparent proxy mode. Then, you need to configure your firewall to redirect all web traffic through the proxy. You can look up the information elsewhere.

Original

From a command prompt:

sudo route add default gw i.p.add.ress eth1

Then, in /etc/network/interfaces, add this line:

up route add default gw i.p.add.ress eth1

Of course, replacing i.p.add.ress with the gateway's IP in both lines.

Edit for new question

If you need to delete a route, just change "add" to "del". Example:

(command) sudo route del default gw 128.0.x.x eth0
(/etc/network/interfaces) up route del default gw 128.0.x.x eth0

You can set a static route for the branch office through the route command as well

route add -net (network_id/prefix) GW (gateway) (interface)