objective-c app update and core data

One thing bothers me, I am developing iOS app. Firstly when hasPerformedFirstLaunch I load core data, it works good. But later, when I'll update my app. I would like to change core data. How can I do this or How you do this? It is an app where a lot of data that will accrue updates.

My first launch

    if (![[NSUserDefaults standardUserDefaults] boolForKey:@"hasPerformedFirstLaunch"]) {
    [self initialCoreData];
    [[NSUserDefaults standardUserDefaults] setBool:YES forKey:@"hasPerformedFirstLaunch"];
    [[NSUserDefaults standardUserDefaults] synchronize];
}