In order to do so, go to the Users section, find the user you want to change and click to edit the account (1).
Click on the little arrow (2) next to the Delete button for the Reset progress option (3) to show up. Click on this option to reset the whole progress of this user.
This action will reset the user's progress to all of the courses he is assigned to. The enrollment date won't be changed.
Note: This option is available to any administrator that has the permission to delete users.