This is a Entity Framework 101 topic. I seriously suggest picking up the books by Julia Lerman(?) on Entity Framework and workign through them.
I have no idea how you got the record from the database, but it should have it's Id (primary key field) number in some field. Use that to lookup the record using the context.
Contact contactToUpdate = context.Contacts.Find(contactId);
contact.FirstName = name.ToString();
... blah blah blah ...
context.Entry(contactToUpdate).State = EntityState.Modified;