You must return to the menu:
- - Roofing and attic
- - - Sloped roof
Choose with or without lost attic space.
You will find your roof in 2D :
Click on the side of the slope to be modified:
Deactivate the symmetry with the icon below:
Then go to the "slopes" menu to change the angle of inclination:
You can either enter the tilt angle directly or click in the angle field and change it with the mouse wheel.