我怎么能将下拉列表默认值更改为另一个值

This code is displaying the dropdown value in textbox on selection. But i want to display my own values in textbox instead of those default values(Default values of dropdown). how could i do that thing . I have a text box with id "#amount".

This is HTML code:

    <select id="d-name" class="form-control" name="menu">
     <option value="Left Chest">Left Chest</option>
     <option value="Simple back">Simple back</option>
     <option value="Complex back">Complex back</option>
    </select>

    <input type="text" id="amount" />

This is javascript code:

 jQuery('#d-name').change(function(e){
     var selectedValue = jQuery(this).val();
     jQuery("#amount").val(selectedValue);
  });

You can use some conditions -

var selectedValue = jQuery(this).val();
var amount = '$0';
if(selectedValue == 'Left Chest')
    amount = '$20';
else if (selectedValue == 'Simple back')
    amount = '$30';
...

jQuery("#amount").val(amount);

You can also use switch cases.

Try this.

<select id="d-name" class="form-control" name="menu">
     <option value="&#36;20">Left Chest</option>
     <option value="Simple back">Simple back</option>
     <option value="Complex back">Complex back</option>
</select>

https://jsfiddle.net/x6f3k165/1/

You can also change the amount by using switch case:

jQuery('#d-name').change(function(e){
var selectedValue = jQuery(this).val();

switch(selectedValue) {
    case "Left Chest":
    amount = "$20"; // amount which u need
    break;
    case "Simple back":
    amount = "Simple back"; // or amount if u need
    break;
    case "Complex back":
    amount = "Complex back"; // or amount if u need
    break;
    default:
    amount = "";
}  

jQuery("#amount").val(amount);
});

<script src="https://ajax.googleapis.com/ajax/libs/jquery/1.2.3/jquery.min.js"></script>
<select id="d-name" class="form-control" name="menu">
    <option value="Left Chest">Left Chest</option>
    <option value="Simple back">Simple back</option>
    <option value="Complex back">Complex back</option>
</select>

<input type="text" id="amount" />

</div>

Try,

<select id="d-name" class="form-control" name="menu">
     <option>Please select...</option>
     <option value="$20">Left Chest</option>
     <option value="Simple back">Simple back</option>
     <option value="Complex back">Complex back</option>
</select>

<input type="text" id="amount" />
<script type="text/javascript">
$('#d-name').change(function(e){
     var selectedValue = $(this).val();
     $("#amount").val(selectedValue);
  });
</script>