The AirPort Extreme will automatically scan to find the best available channel to use at the time, if you simply power it off, what a few seconds, then power it back up again.
If there are a lot of other wireless networks around, you may need to perform this task almost on a daily basis.....since other wireless routers are also scanning to find a good channel to use, and things are changing all the time.
While it is possible to manually choose a Channel....and some users swear by doing this.....how will you know which Channel to choose?
Even if you manage to find a good channel at the time, because other routers are changing their channels, a "good" channel this morning may well be an overcrowded channel this afternoon.
Since you asked how to change the channel, we will answer, but most users will get better results with a lot less effort, if they simply restart their router when the wireless seems to be slowing down.
You do not indicate whether you have a Mac or PC, or you want to use an iPhone or iPad to change the channel. If you have a Mac, we need to know what operating system that it is using. If you are not sure, click the Apple icon in the upper left corner of the screen, then click About This Mac and post back with the OS X Version number that you see there.