I'm trying to "refresh"/"update" a shared string in my app after a user has successfully authenticated. Here's what I have:

```
// MyAuth.h
//@interface
+ (NSString *)sharedAuthToken;

// MyAuth.m
+ (NSString *)sharedAuthToken {
    static NSString *_sharedAuthToken = nil;
    static dispatch_once_t onceToken;
    dispatch_once(&onceToken, ^{
        _sharedAuthToken = [[NSUserDefaults standardUserDefaults] objectForKey:@"token"];
    });

    return _sharedAuthToken;
}

// AuthCtrl.m
- (void)saveOauthInfo:(NSString *)token {

    [MyAuth sharedAuthToken] = token; // getting error here
}

```

I would like to update or refresh the `sharedAuthToken`. How can I do this?

\*Let me know if the code is too confusing or too cluttered..