How does joomla/jomsocial users change their login to use FB ??

No place I can see how my users that are allready registred on the site - can change their login from joomla/jomsocial to use login via FB instead...

For connecting existing accounts, there's a few options:
* Enable the "Automatically Link Accounts by Email" setting in JFBConnect -> Configuration. When enabled, if a user authenticates with FB/G+ and their email address is the same as an existing Joomla account, they'll automatically be linked and logged into that account.
* Disable the Automatic Registration setting in JFBConnect. The user will be brought to a form-based registration flow. They can also login to an existing account on that page instead of creating a new account.
* In the SCLogin module, enable the "Show Connect Account(s) buttons" setting. That will show a link any social networks the user *hasn't* authenticated with when they are logged in. That way, they can login with Joomla and then associate that account to FB/G+.

2. If users that logs in vith FB wants to change to joomla/jomsocial login how do they do that ?

A user created with Facebook will have a Joomla account automatically created for them as well. The username and password should have been emailed to them during the registration process. They can use those credentials to login to that account. Alternatively, they can login using Facebook and go to their profile details (either in JomSocial or the standard Joomla Edit Details view). In there, they can see their username and update their password, if they don't know what it is.

I hope that helps explain, but if not, let us know!

Thanks,
Alex