If you don't have access to the Concept Explorer, with which you select your object and undo shelling by high-lighting the shell feature, right-clicking and "delete feature", you can convert your object to surfaces, delete the inner surfaces and edges, then thicken the outer, or, if it's a closed shell, delete the inner faces and stitch the outers back into a solid that can be re-shelled.

Edited by user Tuesday, June 7, 2016 9:24:57 PM(UTC)  | Reason: Not specified

Select your object, then in the inspector select the right hand icon, Object Properties, this gives you a list of faces and their wall thickness. Selecting one face in the list will highlight that face on your drawing. You can change the wall thickness in the input box above the list. Setting the wall thickness to 0 will give you an open face.