Go into the router settings. Since you don't tell us what kind of router you have, you should Google instructions on how to do this. You can see what channel it's currently using by holding down alt and clicking on the WiFi icon in the menu bar.
While all this stuff is well covered online if you give us the model number we can help. (this is easy enough to do on most models but is different for each model/brand)
While all this stuff is well covered online if you give us the model number we can help. (this is easy enough to do on most models but is different for each model/brand)