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];