I have found the NSNotificationCenter to be particularly helpful for sending information between different ViewControllers.
To simply explain the process, you set up a Notification Listener in the ViewController that you want to be notified, and then you post a notification in the ViewController doing the sending. Something like this to register the listener:
[[NSNotificationCenter defaultCenter] addObserver:self selector:@selector(changeBatteryLabel) name:@"UIDeviceBatteryStateDidChangeNotification" object:nil];
And like this to post the notification:
[[NSNotificationCenter defaultCenter] postNotificationName:@"logOutNotification" object:nil];