Type ./reset-to-factory-anyway (this will reset the Yún/Yún compatible to factory settings) 

Revert boot to internal flash first if the Storage is expanded.

Revert boot to internal flash

reset-to-factory-anyway
reboot



After Yun is completed booted. To reset the OpenWrt-Yun distribution to its default state, press the WiFi reset button (WLAN RST) for at least 30 seconds. The board reverts to the original settings: like just taken out of the box or to the latest update of the OpenWRT image you have reflashed before. Among other things, this removes all installed files and network settings.

https://www.arduino.cc/en/Guide/ArduinoYun