In the model tree, expand the subassemblies that contain the part or subassembly that you want, by LMB on the "+" boxes, until you find the entity you want. Double LMB on the part name and VOLA! The subassembly you want to edit will become active! If you want to open it in another window, do the same thing about expanding parts until you find the one you want then RMB on the part name and LMB "Open".