UIImage showing nil even after initialize it

Question

I declared a UIImage variable :

var s1:UIImage? = UIImage(named: "alphabet O")

I am assigning an another image in a method:

 func setImage(){

    if var img = s1{
    img = UIImage(named: "alphabet X")!
    }

}

The answer can be quite simple but truly I am very new in swift. Please help. Thanks