When stair is selected, Check the properties pallete - Calculation rules, click on the box next to Tread, and a box opens where you can play with different componants. Some are locked but can be unlocked to edit. There are relationships that must be kept.
The stair object is not a simple object but take a look in the style under Design Rules and see if the "Use Rules Based Calc" is checked. If yes then you are restricted to conform to those rules.
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