To restart the the Arduino, tap the button next to the Ethernet port two times.
To reset the WiFi press the button located next to the USB-A connector, for longer longer than 5 seconds, but less than 30. The Yún will start its own wiFi network Arduino Yún-XXXXXXXXXXXX. Any other modification/configuration will be retained.
To reset the Linino distribution to its default state, press the WiFi reset button for at least 30 seconds. The board reverts to the original settings as if it had been reflashed or taken out of the box. Among other things, this removes all installed files and network settings.