There's a couple of different ways to do this. One way is to Edit the Profile of your wall. When you select the wall your ribbon menu at the top of your screen will change. There should be a button called "Edit Profile." This feature essentially allows you to sketch the boundary of your wall into some odd shape. In your case, you could align the top boundary of your wall to match the slope of your roof diaphragm or the framing. You can also adjust the height of the top of the wall as needed.
Another way to accomplish this would be to put a Reference Plan in your model that matches the slop of the roof or the slope of a framing member. When you select the wall there should be another button in the ribbon menu "Attach Top / Base." Please note that there is a toggle when you are in the command that identifies if you want to modify / attach the top of the wall, or the base of the wall. Make sure you are have "Top" checked, and then select the reference plan you inserted into the model. The top of the wall should adjust to match the slope of the reference plane, as well as extend up to the reference plane. If you need to adjust the elevation of the top of the wall, edit the elevation of the reference plane and the top of the wall will follow. If you change your mind, and don't want the sloped top of wall, select the wall and pick the "Detach Top / Base" button and then click the "Detach All" button on the left of the ribbon menu. This will revise the wall to match its original position.
Please note that you cannot attach the top of a single wall element to 2 different slopes. You can either split the wall where the change in slope occurs, or you can Edit the Profile. Another thing to note in regards to the reference planes... when plotting, make sure your Setup is such that Reference Planes are checked off so they do not plot. Hope this helps.
