To change the wireless channel on the ZyXEL VMG1312 you will need to access the router interface through a web browser. To do this make sure you're on a device that is connected to the router, open a web browser, and then type the IP address of the router (192.168.1.1 by default) into the address bar then press enter. You will then be asked to log into the router.
By default the username is admin and the password is 1234, but the password should have been changed during the setup of the router.
Once logged into the router hover over the "Network Setting" option on the bottom menu and the select the "Wireless" option from the popup menu.
Once in the wireless section of the router you will see "Channel" under the heading "Wireless Network Setup". By default the channel is set to "Auto", you can click this to bring up a dropdown menu that will allow you to select the channel manually.
Once you've made the changes you wish to make scroll down to the bottom and click "Apply".
If you're unsure which channel to select and the automatic channel seems to be causing issues then the router has a built in wireless channel scanner. To use this feature you must set the wireless bandwidth to 20MHz.
Once you have set the bandwidth to 20MHz scroll down to the bottom and click "apply", then click "Channel Status" on the top menu.
Once on the "Channel Status" section click "Scan".
Once it has been scanned you will see a chart and the noise on each of the channels so you can select the channel with least interference.