To Edit a 3D Primitive Element

Select the Edit 3D Primitive tool.
Select the 3D primitive.
The element highlights.
Accept the element.
A dialog opens with the editable dimensions displayed, showing the existing values.
Make changes, as required, to the dimension(s).
Click OK.
The 3D primitive is modified to match the new values.

Note: If you extrude a block, it is recognized as a slab by the Edit 3D Primitive tool, thus letting you edit the Length, Width, and Height (not just the extruded distance).