Programmatically Change multiplier of Alignment Constraint of Center X / Y

So for Y, if you set the top of the image equal with a constant of 0 to the top of the superView. then enter this code:

@IBOutlet weak var topc: NSLayoutConstraint!


let heightOfSuperview = self.view.bounds.height
topc.constant = heightOfSuperview * 0.90 // this has the same effect as multiplier  

This is equivalent of Center.y multiplier = 0.9:1