How can I switch views programmatically in a view controller? (Xcode, iPhone)

If you're in a Navigation Controller:

ViewController *viewController = [[ViewController alloc] init];
[self.navigationController pushViewController:viewController animated:YES];

or if you just want to present a new view:

ViewController *viewController = [[ViewController alloc] init];    
[self presentViewController:viewController animated:YES completion:nil];

If you want to present a new view in the same storyboard,

In CurrentViewController.m,

#import "YourViewController.h"

UIStoryboard *storyboard = [UIStoryboard storyboardWithName:@"MainStoryboard" bundle:nil];
YourViewController *viewController = (YourViewController *)[storyboard instantiateViewControllerWithIdentifier:@"YourViewControllerIdentifier"];
[self presentViewController:viewController animated:YES completion:nil];

To set identifier to a view controller, Open MainStoryBoard.storyboard. Select YourViewController View-> Utilities -> ShowIdentityInspector. There you can specify the identifier.

Swift version:

If you are in a Navigation Controller:

let viewController: ViewController = self.storyboard?.instantiateViewControllerWithIdentifier("VC") as ViewController
self.navigationController?.pushViewController(viewController, animated: true)

Or if you just want to present a new view:

let viewController: ViewController = self.storyboard?.instantiateViewControllerWithIdentifier("VC") as ViewController
self.presentViewController(viewController, animated: true, completion: nil)