How to segue back to a UIViewController that’s already loaded?

Assuming by your use of “push” you mean that you are using a UINavigationController, then you can use the following to return to the top of the stack.

[self.navigationController popToRootViewControllerAnimated:YES];

or

UIViewController *prevVC = [self.navigationController.viewControllers objectAtIndex:<n>];
[self.navigationController popToViewController:prevVC animated:YES];

to pop to a specific level where <n> is the level.

or

[self.navigationController popViewControllerAnimated:YES];

If you just want to pop one level back up the navigation stack.