@IBOutlet weak var batsmenScoreStepper:UIStepper!   
@IBAction func resetScoreButton(_ sender: Any) {
   batsmenScoreStepper.value = 0.0;
   displayBatsmenOneScoreLabel.text = "\(batsmenScoreStepper.value)"
}
you should first take outlet of your UIStepper and reset it. 
1
solved Reset Score Button iOS