Step 1: Open the AirPort utility on a computer that is connected to a network with an AirPort Extreme or Time Capsule router. The utility is located in the Utilities subfolder of the applications folder.
Step 2: Click on your AirPort Express router in the list of devices then click "Edit".
Step 3: Click "Wireless" then click on the Network Mode pop-up menu so that you can select the "Extend a Wireless Network" choice.
Step 4: Click the "Wireless Network Name" pop-up menu and select the name of the network that you want to use your AirPort Express to extend.
Step 5: Log in to the network by entering its username and password. Your AirPort Express is now a range extender. You can close the AirPort Utility if you wish.
Using multiple Wi-Fi base stations wirelessly to extend the range of an AirPort network over a wider physical area when the range of a single base station is insufficient.
A network that utilizes more than one Wi-Fi base station to extend the range of a network or to extend features such as Internet access music streaming printing storage etc.
The Wi-Fi base stations may be connected together via Ethernet or wirelessly.
How To Use Airport Express to Extend WiFi?