The stair tool allows you to design with freedom, one of the key features of it is to edit the boundaries.
Clicking in one of the external side edges of the stair will highlight both sides green.
(If you click on the baseline it may highlight it blue, just de-select and click on the other side edge)
After highlighted, click on the edge you want to edit and use the pet palette to modify it.
Change the boundaries according to your design and see the results in 3D.
