// Gott some info from the server
NSMutableArray *serverContacts
for(NSDictionary *jsonContact in contactsFromServer){
    Contact *myContact = [Method to convert jsonContact to a "Contact" object];
    [serverContacts addObject: myContact];
}

NSFetchRequest *blabla;
NSArray *results = // execute request, returns an array of "Contact" objects as well

for(Contact *contact in results){
    contact.firstName = "blablabla"; // Update some field
}

[managedObjectContext save]; // Save context, would save the updates, but also insert all of the objects from the "serverContacts" array which I just wanted as regular objects for reference and never wanted to insert them.