To change radio channels on the AirPort Express Base Station (AX), using the AirPort Admin Utility, either connect to the AX's wireless network or temporarily connect your computer directly (using an Ethernet cable) to the Ethernet port of the AX, and then, try these settings:
AirPort tab o Channel: <select the desired radio channel, automatic is the default>
o Click on "Update" to save the settings to the AX.