Creating a logout Button for Photo Bombers App.. How to do this??

<p>
//...
// ViewDidLoad in PhotoViewController.m
- (void)viewDidLoad {
    [super viewDidLoad];

    self.title = @"Photo Bombers";

    // Creating Logout Button
    UIBarButtonItem *logout = [[UIBarButtonItem alloc]initWithTitle:@"Log Out" style:UIBarButtonItemStylePlain target:self action:@selector(logout:)];
    self.navigationItem.leftBarButtonItem = logout;


    [self.collectionView registerClass:[JWFPhotoCell class] forCellWithReuseIdentifier:@"photo"];
    self.collectionView.backgroundColor = [UIColor whiteColor];

    [self checkForAccessToken];

}
//...

// Methods to be noted:
- (void)checkForAccessToken {
    NSUserDefaults *userDefaults = [NSUserDefaults standardUserDefaults];
    self.accessToken = [userDefaults objectForKey:@"accessToken"];

    if (self.accessToken == nil) {
        [SimpleAuth authorize:@"instagram" options:@{@"scope": @[@"likes"]} completion:^(NSDictionary *responseObject, NSError *error) {

            self.accessToken = responseObject[@"credentials"][@"token"]; 

            // Setting userDefaults to the accessToken.
            [userDefaults setObject:self.accessToken forKey:@"accessToken"];
            [userDefaults synchronize]; // Saves user defaults.

            [self refresh];
        }];
    } else {
        [self refresh];
    }
}

- (void)refresh {
    NSURLSession *session = [NSURLSession sharedSession];
    NSString *urlString = [[NSString alloc] initWithFormat:@"https://api.instagram.com/v1/tags/photobomb/media/recent?access_token=%@", self.accessToken];
    NSURL *url = [[NSURL alloc] initWithString:urlString];
    NSURLRequest *request = [[NSURLRequest alloc] initWithURL:url];
    NSURLSessionDownloadTask *task = [session downloadTaskWithRequest:request completionHandler:^(NSURL *location, NSURLResponse *response, NSError *error) {


        NSData *data = [[NSData alloc] initWithContentsOfURL:location];
        NSDictionary *responseDictionary = [NSJSONSerialization JSONObjectWithData:data options:kNilOptions error:nil];

        self.photos = [responseDictionary valueForKeyPath:@"data"];

        dispatch_async(dispatch_get_main_queue(), ^{
            [self.collectionView reloadData];
        });
    }];
    [task resume];
}

// This is where I am implementing my method for logging out the User.
- (IBAction)logout:(id)sender {
    self.accessToken = nil;
    [self checkForAccessToken];
    NSLog(@"Access Token: %@", self.accessToken);
}

</p>