Open the Config Tool and find your device, then click continue.
Click on the Reset settings button. Then click Load Defaults. This will restore the config back to how it was when you first programmed the Arduino with the Config Tool.
Resetting to Arduino Defaults
Open the Config Tool and find your device, then click continue.
Click on the Restore Device back to Arduino button. This will restore the device back to a regular Arduino so it can be used in the Arduino IDE again.