Follow these steps:

1.Plug the AirPort Express into the wall and wait until it boots up.

2.Go into your Wi-Fi settings on your iPhone, iPad, or Mac and join the new AirPort network.

3.Click on the Other Options button.

4.Select “Add to an existing network.”

6.Choose your existing Wi-Fi network name from the list. Enter the password.

AirPort Express Setup | Apple AirPort Express Setup