To un-assign users from a course, you first need to go to the Assigned By Me page in the Learning module. Click on the course you want to un-assign users from. The Course Dashboard opens up. There, go to the Learners tab. Here you will see the users who have been assigned the course. Under actions, you can find the Un-Assign button.
Click on the button to un-assign a user from a course. Keep in mind, there’s no way to mass un-assign users from a course. Un-assigning users resets a user’s progress so it is usually something that must be done before a user begins a course.