How do I change my MTU settings on a Linkysys BEFW11S4 (ver. 4)?

I recently got a Xbox 360 that is connected to my router wirelessly and I have a little brother with a Xbox 360 that is connected to the router directly with a wire. We both keep getting disconnected from Xbox Live and it is getting pretty annoying. I called customer support and they told me to open some ports, I did... Then it still had that problem, and now they are telling me to change my MTU settings to 1365 or higher. How can I do that? Please help, much appreciated! =]