Mirror PropertyManager

To open the Mirror PropertyManager:

In an open sketch, click Mirror Entities (Sketch toolbar) or Tools > Sketch Tools > Mirror.

Options

Entities to mirror Select some or all entities to mirror.
  Copy Select Copy to include both the original and mirrored entities. Clear Copy to include only the mirrored entities.
Mirror about Select any line, the linear edge on a model, a reference plane, planar model faces, or the linear edge on a drawing about which to mirror.