I found this using google. You need to use the utility, you can't use a web browser for airports (unlike other routers.)
To change the channel, follow these steps:
Open the AirPort Admin utility (found in the Utilities folder within the Applications folder).
Select your base station in the resulting window and click Configure. If asked to, enter the password.
Click the AirPort tab and choose a new channel from the Channel pop-up menu and click Update.
Ideally, the channel you choose will be at least four channels away from an adjacent wireless network or phonefor example, if a nearby network is broadcasting on channel 1, youll choose channel 6 for your network. Of course, because you have no control over those other networks in your complex you dont know which channel theyre broadcasting on. For you, trial and error is the best way to go (unless you know your neighbors well enough to ask).