How do I change the letter spacing in a UILabel?

The easiest way is to create a custom UILabel class and set the letter spacing from Storyboard.

open class CustomLabel : UILabel {
    @IBInspectable open var characterSpacing:CGFloat = 1 {
        didSet {
            let attributedString = NSMutableAttributedString(string: self.text!)
            attributedString.addAttribute(NSKernAttributeName, value: self.characterSpacing, range: NSRange(location: 0, length: attributedString.length))
            self.attributedText = attributedString
        }

    }
}

You can use the NSKernAttributeName attribute on an attributed string:

UILabel *label = [UILabel new];

NSMutableAttributedString *text = [[NSMutableAttributedString alloc] 
                                   initWithString:@"127"];

// The value paramenter defines your spacing amount, and range is 
// the range of characters in your string the spacing will apply to. 
// Here we want it to apply to the whole string so we take it from 0 to text.length.
[text addAttribute:NSKernAttributeName 
             value:@-0.5 
             range:NSMakeRange(0, text.length)];

[label setAttributedText:text];