How to change which Aiport Express a MBP is connected to?
There are multiple ways to change which access point you connect to.

Using the GUI

Open network in your system preference
use the Join other networks and find the airport Express in your WiFi choices
check the Automatically join this network.
click on Advanced and move that access point to the top of your list.

If for some reason this does not work then use the Terminal

There are 2 or more ways to select the network using Terminal.

There is the ifconfig command line, which is in depth network setting, I would not use that one right now.

the other is Networksetup command line which is replacement for the GUI method mentioned above.

You would use the output from the airport command and the networksetup utility to adjust properties such as connecting to a new Wi-Fi network. 

First, scan for networks with the "airport -s" option (you can skip this if you know your SSID you want to use.

and then use the SSID of the listed networks with the 

networksetup -setairportnetwork &lt;device name&gt; &lt;network&gt; [password] command.

 should be the "en0" which is your WiFi

or

networksetup -switchtolocation "location name"