Are you asking about a drawing section view or a 3D section? In either case, you are able to modify the section line (plane).
In the case of a drawing view, you double click on the section line and a stripped down sketcher workbench opens. Then you modify the section line as needed.
In the case of a 3D section, you can edit the section plane by dragging the 6 degree of freedom icon that appears on the section view or manually enter parameters for the section plane.
In both case, I find manipulating the sections after creation to be a little bit clunky from a UI perspective. Just keep poking at it and read the docs. It will eventually make sense.
