Open AirPort Utility on your Mac or iOS device, then follow these steps to restart your base station. If you have more than one base station, restart your primary base station last.
1. Select your base station in AirPort Utility, then enter the base station password, if prompted.
2. On Mac, choose Base Station > Restart from the menu bar.
3. On iOS devices, tap the base station in AirPort Utility, then tap Edit > Advanced > Restart Base Station.
If you can't restart using AirPort Utility, just unplug the base station for a few seconds, then plug it back in.

AirPort Express Setup | Reset AirPort Express | Apple AirPort Express Setup