You shouldn't be trying to - that isn't what the app delegate is for. Or, depending on the task, the state of the switch should be stored to user defaults or something like that.
If the view controller should do that job, it can do this by observing the UIApplicationWillResignActiveNotification
notification (if it really needs to be done then).
If the app delegate should do the job then the view controller should update the value in NSUserDefaults
whenever the switch is changed.