A video which shows the procedure can be found at the bottom of this article
Open the user by going to Administration -> Users.
Click on the File button (upper left corner) -> Reset password:
When the "Reset password" button is clicked, the password goes back to being the same as the user name.
Also, the "Must change password next login" check box will be ticked on meaning that when you log in with this user again, you will be prompted to set a new password.