There is no standard way, so it is very difficult to make a manual for this.
To change the channel on your router/access-point, you usually have to do these steps.
- Find out the IP-Address/name of your router/access-point
- Enter it in the address-bar of your browser
- Enter the credentials (usually it is username & password which you may find on the device itself or in the manual, in case you've never changed it)
- Find the setting for the channel, password, ...