You can change the height of a single wall by switching the wall out for a Straight Half-Wall. (More info can be found here https://www.homedesignersoftware.com/support/article/KB-02969/changing-wall-heights.html)
It sounds like you're trying to make a Shed Roof design which is much easier to achieve with our Automatic Roof tools. Here's a video to give more information on that if that's what you're looking for! (https://www.chiefarchitect.com/videos/watch/6125/shed-roof.html)
ConnorE's post in Change wall height? was marked as the answer