override func tableView(tableView: UITableView, didSelectRowAtIndexPath indexPath: NSIndexPath) {
 var cell:UITableViewCell = tableView.cellForRowAtIndexPath(indexPath)!
  //you got your cell now suppose your button tag is 3
 var button:UIButton = cell.contentView.viewWithTag(3) as! UIButton
 //you got you button now change the image of the button its done
}