Select Mirror from the Transform options to display the Mirror toolbar:
When you mirror a workplane, the Y axis is flipped to ensure the creation of a mathematically correct axis.
Mirroring in YZ 
converts this:
to this:
Use the options on the toolbar as follows:
Mirror in XY — Click to mirror the entity in the XY plane.
Mirror in YZ — Click to mirror the entity in the YZ plane.
Mirror in ZX — Click to mirror the entity in the ZX plane.
Mirror in line — Click to mirror the entity in a plane defined by selecting either an existing line or two points.
Workplane origin — When selected, the active workplane is the origin. If no workplane is active then the global coordinate system is the origin.
Bounding box origin — When selected, the origin is the centre of the bounding box containing all the entities.
Move origin — When selected, you can move the origin graphically by dragging or by entering coordinates using 
, 
, or 
and 
in the Status bar.
Finish — Click to accept the changes and close the toolbar.