To make changes to an existing course in Schoolrunner, use the Quick Search (the magnifying glass at the top of any page) to find the course and navigate to its page. Then select "Edit".
From this screen, you'll be able to make various changes to the course. However, beware of changing the course name if this course is synced in from your SIS. Changes to the name may throw a wrench in Schoolrunner's sync and generate duplicate courses.
This screen is where you can add or remove sections that are linked to the course. Sections are groups of students who are all enrolled in the same course. If a student has assessment results for this course but doesn't have a grade for it, it may be caused by him/her not being enrolled in a section that's linked to the course. This is the place to rectify that.
Learn more about the other fields here.