Factory Resetting your NextBox will inevitably delete all your files on the NextBox and any configuration settings.
To factory reset your NextBox simply press the hardware button for 5 seconds.
The NextBox LED will change to blue once you press the button, after the 5 seconds have passed the color will change to blinking red (see LED documentation). Once the LED has finally reached green again your NextBox is ready for the initial setup as described in Getting Started
The hardware button is a pinhole-style button similar to other embedded devices. In order to press it, it is recommended to use a thin (~1mm) needle-like pin. E.g., the pin to remove a sim-card from a smartphone is working great. Further take care to not press the button too hard, be careful, there should be a recognizable haptic feedback (button click) with minimal applied force.