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).