You can assign and unassign courses in catalogs, learning plans and channels directly from the course details in Learning Setup > Courses & Learning Plans > Courses.
To do so, click the plus button in the top right corner of the screen and select Assign to Catalog, Assign to Learning Plan, or Assign to Channel. Make your selection in the panel and click Confirm.
The course associations to catalogs, learning plans and channels are available in the corresponding tabs of the course edit view. From the Learning Plans, Catalogs, and Channels tabs, you can check any associations already available for the selected course, or remove existing assignments.
To do so, click the ellipsis menu at the end of the course row and select Unassign.