If you create a custom stair with the tool and select it (in 2D or in 3D), the general opening methods for library parts should work. The “Open object” command opens all stair-related library parts of the placed element (multiple ones). If you wish to open just one single object, you can use the Report window: type the exact name of the libpart you wish to edit, select the text and use the “Open object” again (or the hotkey).
Dominika Bobály
Library Developer, Library Team
GRAPHISOFT SE