Use following code to change default quantity input boxes into dropdown select options.

Method:

1. Copy code to your functions.php.

/* Change Product Quantity Input to Dropdown */
function woocommerce_quantity_input() {
 global $product;
  
 $defaults = array(
  'input_name' => 'quantity',
  'input_value' => '1',
  'max_value'  => apply_filters( 'woocommerce_quantity_input_max', '', $product ),
  'min_value'  => apply_filters( 'woocommerce_quantity_input_min', '', $product ),
  'step'   => apply_filters( 'woocommerce_quantity_input_step', '1', $product ),
  'style'   => apply_filters( 'woocommerce_quantity_style', 'float:left; margin-right:10px;', $product )
 );
 
 if (!empty($defaults['min_value']))
  $min = $defaults['min_value'];
  else $min = 1;
  
 if (!empty($defaults['max_value']))
  $max = $defaults['max_value'];
  else $max = 20;
  
 if (!empty($defaults['step']))
  $step = $defaults['step'];
  else $step = 1;
  
 $options = '';
 for($count = $min;$count <= $max;$count = $count+$step){
  $options .= '' . $count . '';
 }
 
 echo '
' . $options . '
';
}