To un-assign users from a course, first you would need to go to the Authored by Me or Assigned By Me tab in the Learning module. Click on the course you want to un-assign users from. 

The Course Dashboard opens up. Please 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. Please note that un-assigning users resets a user's progress and takes it off from his/her assigned course list, so it is usually something that must be done before a user begins a course.