Mirror V

This mirrors the selected UV elements (points, edges, or polygons) vertically. The origin of the mirroring is located at the origin of the modeling coordinate system.

If nothing is selected, the entire contents of the UV tile are mirrored vertically, relative to the center of the texture tile.
Alternatively, you can use Mirror Tile V. In this case, the mirroring is always based on the center of the UV tile, even if the command is applied to a selection of UV elements.