How to set back button text in Swift

The back button belongs to the previous view controller, not the one currently presented on screen.
To modify the back button you should update it before pushing, on the view controller that initiated the segue:

override func prepareForSegue(segue: UIStoryboardSegue, sender: AnyObject?) {
    let backItem = UIBarButtonItem()
    backItem.title = "Something Else"
    navigationItem.backBarButtonItem = backItem // This will show in the next view controller being pushed
}

Swift 3, 4 & 5:

override func prepare(for segue: UIStoryboardSegue, sender: Any?) {
    let backItem = UIBarButtonItem()
    backItem.title = "Something Else"
    navigationItem.backBarButtonItem = backItem // This will show in the next view controller being pushed
}

OR

// in your viewDidLoad or viewWillAppear
navigationItem.backBarButtonItem = UIBarButtonItem(
    title: "Something Else", style: .plain, target: nil, action: nil)

You can do it from interface builder as follows:

click on the navigation item of previous view controller

from the attributes inspector set the back button text to whatever you want. Thats it!!