To boot into Safe Mode on the Fire TV Cube, disconnect the power cable, press and hold the Volume Down button on the top of the Fire TV Cube, and connect the power cable. Keep holding the Volume Down button until you see “Safe Mode” appear in the lower left corner. It should appear just after the “Fire TV” boot animation disappears. To exit Safe Mode, simply restart the Fire TV Cube by going to Settings > Device > Restart
How does this “factory reset” the cube? Looks like only a partial explanation.
Thank you Pete for reaching out. If can access settings menu, you can reset it.
Go to Settings.
Select My Fire TV.
Select Reset to Factory Defaults.