The Flip command lets you easily change the orientation of UV shells within the UV Editor. Flipping a UV shell is useful if you need to correct a texture that is reversed (see Display UV winding order).
To flip UVs on a UV shell
The selected UVs are flipped in the specified axis.
It is easiest to see the results of a flipped shell if you first turn on Shaded Mode and set it to show Front and Back Facing shells.