It seems to be set at 17 but I can't find a way to change it
Autocad help is no help at all as it says change the field to a user defined one, whilst not actually telling you how to do that
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.