You need to create an IBOutlet of your constraint.
Then you set the constant value of your constraint in code:
labelWidthConstraint.constant = newValue
If you want it animated you can do something like this:
labelWidthConstraint.constant = newValue
UIView.animate(withDuration: 0.3, animations: {
self.view.layoutIfNeeded()
})
self.labelWidthConstraint.constant = newValue;
[UIView animateWithDuration:0.3 animations:^{
[self.view layoutIfNeeded];
}];