To edit a clip plane, select its node in the object tree and edit the properties.
Alternatively, to modify the planes visually, right-click the Clip Planes manager node and select Edit Clip Planes.
An interactive in-place dialog will appear in the scene. (If the scene display is not in view, the Edit Clip Planes action will cause the display to be opened or become active.) Tick the Display Box Tool checkbox to activate a tool for modifying the planes.
Use the 
(Up-Arrow) button to minimize the in-place dialog to see the entire scene.
You can adjust the tool by clicking and dragging any of the end point handles of the lines, or any of the sides of the bounding box.
When done with the tool, restore the dialog with the 
(Down-Arrow) button. Click Apply to implement your changes. If you have no further changes to make with the tool, click Close to remove the dialog.