set segue.destinationViewController property

method 1. typecast

APPDetailViewController myController = (APPDetailViewController)segue.destinationViewController;
myController.myString = string;

method 2. selector

- (void)prepareForSegue:(UIStoryboardSegue *)segue sender:(id)sender
{
    if ([segue.destinationViewController respondsToSelector:@selector(setMyData:)]) {
         [segue.destinationViewController performSelector:@selector(setMyData:) 
                                               withObject:myData];
    } 
}

in destination View Controller

@property (nonatomic, strong) MyData *myData;