The channel selection works by giving the WIFI particles a certain temperature with the PROP tool. Each channel has a band of 100 C so it's easiest to start with temp 0 for the first channel, temp 100 for the second channel, temp 200 for the third channel etc.

So let's say you have 2 particles of WIFI and want them to communicate on the 12th channel: activate the PROP tool, select temp in the top menu, type 1100 and click OK. Now click both the WIFI particles to give them the property you just set, and that's all.

The easiest way is just to use HEAT / COOL tools. Look at the tmp value while changing the temperature to see the current channel 

Some scripts allow you to change the channels of wifi immediately without console commands or property tools.