Let $ z = \cos \frac{2\pi}{7} + i \sin \frac{2\pi}{7} = e^{i \frac{2\pi}{7}} $. Then its conjugate is $ \overline{z} = \cos \frac{2\pi}{7} - i \sin \frac{2\pi}{7} = e^{-i \frac{2\pi}{7}} $.