Depending on your school's set-up, you might not be able to "leave" a course. However, you can still remove the course from your dashboard! On the course card on your dashboard, select the three dots in the upper right, then select unfavorite. Here's the guide that shows how to unfavorite.
Just for reference, courses may stay on your dashboard longer than intended if the instructor didn't set an end date for the course. You can usually reach out to them or your school's eLearning/IT team and they can set a course end date so it stops appearing on everyone's dashboards.
