You can change the shape of a 3D freeform space by in-place editing the surfaces of the space.
Note: When you start an in-place edit operation on an extruded 3D space, it is automatically converted to a 3D freeform space. You can convert it back to an extruded 3D space once you have finished editing it, but you will loose any geometry changes that are not compatible with extruded 3D spaces.