At the main FTP menu is a list of all FTP accounts.  Click the "change" link next to the account you wish to alter.  You must enter a password twice, and this password can be your old password or a new one.  You may also change the access level of the account (see above). To change the password for the main username (eg: sitehelp), it's linked with your main DirectAdmin login/password, so you only need to change your DirectAdmin password to change the ftp password for that 1 account.