Revit Architecture
Revit MEP As you place a compound wall in a plan view, you can press the Spacebar to flip its orientation. You can also select an existing wall in a plan view, and control arrows appear:
. If you click on the arrows, the first and last layers of the wall change position.

Compound wall as seen in plan view before flipping

Compound wall after flipping (the first and last layers have changed position)