Open the settings for the Stair and select "Edit this Stair" from the tiny little pop up menu, in the 2d plan view Stairmaker shows of the stair there are break lines shown, if you float your mouse over the ends of the breaklines the mouse will turn into a crosshair and you can then drag the break lines to the tread you want.
AC versions 3.41 to 25 (UKI Full 5005).
Using AC25 5005 UKI FULL
Mac OSX 10.15.7 (19G2021) Mac Pro-2013 32gbRam AMD FirePro D500 3072 MB graphics