Ok, so you can look up a customer (or Contact's) login email and change their password for them from the Contacts section of your admin interface. This process will give you the option to send an email notification to their registered email address notifying them of the new password.
You can also hit the Edit button to the top right of that page to update any of their other account details.
The customer can then log into their live site account: http://yourdomain.com/login
..and edit their own details from their MyAccount page: http://yourdomain.com/myaccount