Click on Admin icon at the sidebar on the left. By default sub-menu Departments belonging to Groups is highlighted
Click on Users & Roles from the top menu. All the users created are listed here.
Ensure that there are at-least two users in an Account. Select an User from the list. 
For the highlighted user , click on delete icon at the right
Click on the button Delete User.
If the deletion of User was successful a green banner appears at the bottom right.