Change channel in Airport Express when extending network I have a Airport Time Capsule running my WiFi network at home, but have a bit of a weaker area near my bedroom (upstairs) so got an Airport Express to help it out.  I set it up using Airport Utility to extend the network, and that seems good so far.  It's connecting wirelessly, for now, which I'm comfortable with (not concerned with actual downstream speed, at the moment).

Unfortunately, the Airport Express seems to have picked a poor channel.  I used NetSpot to see how its coverage was, and the answer was more-or-less terrible: while the Time Capsule still is in the green-ish (~40-50 db) range even in the 'poor' areas, the Airport Express is twenty feet away and still no better than that.

I notice from the channels that the Express is on the very overused near me Channel 11, while the main Time Capsule is on Channel 1 (both for 2.5Ghz, which is the one I'd expect it to mostly use).  Is there a way to change that?  I go into Airport Utility, and I can change the channel the Time Capsule is using (under Wireless Options), but that setting isn't there for the Airport Express.  My understanding of how a Wi-Fi Extender should work is that it should not use the same channel; so I don't see why Apple would force this to automatic.  Is it so (forced) or is there a way to work around this?  (Also, the 5Ghz channel is identical for both devices, which again I'd think is bad.)

I have both Mac and Windows computers available, if that's relevant.
You've identified the big problem with WiFi backhaul. You have to have both radios on the same channel and half the time the radios are sending traffic between each other and the other half talking to clients.

Since wifi is half duplex and everyone has to wait for the last person to finish talking before the next can speak - you run into serious limitation once you're at the 10 to 30 client number.

For almost all cases where you want channel diversity - run a wire so that your backhaul is over power line / ethernet / coax cable - anything but WiFi will make your network so much faster and easier to manage.

For corporate settings, you almost need a third radio dedicated to the backhaul to make most mesh networks work best or bite the bullet and run wired backhaul there as well. Normally power over ethernet means wired backhaul is doable for large deployments as well as for small home / office situations.