Extend an existing Wi-Fi network in AirPort Utility on Mac
You can extend your existing Wi-Fi network with an additional base station using AirPort Utility on your Mac or AirPort Utility for iOS on your iOS device (see About AirPort Utility for iOS). After extending your Wi-Fi network, your network uses the existing network settings and broadcasts to a larger area.
Physically connect the base stations to each other
Verify that the power lead is connected to the Power port
on the primary base station and plugged into a power point.Plug one end of an Ethernet cable into the WAN port
on the base station you’re adding to the network, and the other end into a LAN port 
on the primary base station.If you don’t physically connect the base stations using an Ethernet cable, you’ll connect wirelessly during the setup process.
Note: Using an Ethernet cable to connect the base stations provides the best throughput and is thus preferred over connecting wirelessly.
Extend your Wi-Fi network using your Mac
On your Mac, click the Wi-Fi icon
in the menu bar, then choose the base station listed under New AirPort Base Station.Follow the onscreen instructions.
AirPort Utility Setup Assistant automatically provides the best solution for extending your network.
If you want to set up your network using different settings, click Other Options.
Extend your Wi-Fi network using your iOS device
On your iOS device, go to Settings > Wi-Fi, then tap the name of your base station (for example, “AirPort Extreme 0affff”) listed under “Set up new AirPort base station”.
Follow the onscreen instructions.
AirPort Utility for iOS Setup Assistant automatically provides the best solution for extending your network.
If you want to set up your network using different settings, tap Other Options.
Detailed information about wireless networking, including a variety of support articles and a PDF of the setup guide that came with your base station, is available on the AirPort Support website.