What you need is to check out the User Profile Service Application area.
- Go to SharePoint Central Administration
- Go to Manage Service Applications
- Select User Profile Service Application
This area is the home of the accounts that are in SharePoint, as well as all the information about the users and how it is displayed on their "My Site"/profile page areas.
If you select Manage User Profiles, this is the area where you can override information from Active Directory about users that are in sharepoint. If you connect to AD, you'll see a little database icon next to the fields that indicates it is synchronized. Even if you use AD, these field can be individually overriden with new information. If you don't have AD, then these will all need to be populated manually by you.
If you go to Manage User Properties you will find all the options to show which fields are editable and which fields are not on a user's profile. This includes, but is not limited to:
- Email
- Display Name
- Homepage/Url
- Department
- Manager
If you have AD, you'll rarely visit this service admin page, because everything will just work. If you don't, then you'll need to check out this area to change the information about the people that are using your SharePoint instance.
Hope that helps!