To manage your own or other accounts, you first need to authenticate. Click the Unlock button in the top right corner and type your password. Note that you are required to be an administrator. If you're the only user on the system, then your account is an administrator by default.
Upon clicking to remove the user, you will be brought to the following window. In this window, you have the option to keep their files, delete their files, or cancel the removal of the user’s account.