Mirror Trim
shortcut DM in the UV editor
The Mirror tool is used to mirror Trims in UV space, across a Trim's local U or V axis.
Keep in mind
Mirroring Trims like this should only be necessary if you utilize Parallax on your Trim Sheet Material.
Selection
Faces with a Trim Sheet Material applied.
Modes
ALT: Batch Mirror separate selection islands
CtRL: Mirror accross V instead of U