Press and Hold on the Top Button

While still holding the top button, press and hold on the small back button.

This button is slightly recessed. You'll likely need to use a paperclip, SIM Eject tool, or other small pinpoint to press this button.

Upon pressing and holding the back button you'll feel a short vibration, continue to hold both buttons until you feel a longer vibration which indicates the factory reset is complete. After this you can release all buttons and re-pair it to your device.