If you want to assign courses to a group, you can do so by clicking on the icon under the Courses column on the Manage Groups page. You will be redirected to the courses listing page of the group. On the courses listing page, press the plus button at the top. You can select courses from the list. Press ASSIGN button to assign courses to the group. This will assign the selected courses to all the users in the group.
You can unassign courses from a group. This can be done by selecting the courses which you want to unassign on the courses listing page of the group. Press UNASSIGN button to unassign courses from the group. This will just remove these courses from the group and have no impact on the users who have already been assigned these courses on account of them being a part of the group.