I don't know anything about the airport express, but if its like normal routers, to get into the settings, go to the PC connected to the router, and open a web browser and type 192.168.0.1 This should open the routers setup interface. Now have a look around for something like wireless settings (on my router, a D-Link, its under the setup tab). In wireless settings, there should be an option to select what wireless mode you want. If none of this works, apple must have a different setup method, in which case you'll have to have a look at the manual to see how to set it up.