jQuery Ajax call

Hello,

I’m making my first custom Ajax call in a single.php theme file.
here’s my js code:

jQuery(document).ready(function(){

		                        jQuery("#mp3file").live("click", function(){
    jQuery.ajax({
	type: "POST",
	url: "<?php bloginfo('wpurl') ?>/wp-admin/admin-ajax.php",
	data: "action=update",
	success: function(data){
        },
        complete: function(data){
	     jQuery("#current_available_dl").html(data);
	}
     });
    });
});

This is my code in the function.php file:

function update_available_downloads(){

        global $post;
        $custom = get_post_custom($post->ID);
        $current = get_available_downloads($post->ID);

        $new_value = $current - 1;

        update_post_meta($post->ID, "current_amount_of_downloads", $new_value);

        return $new_value;

    }

    add_action('wp_ajax_update', 'update_available_download');
    add_action('wp_ajax_nopriv_update', 'update_available_download');

Don’t see what I’m doing wrong here?
If anyone can help me out here?

Thanks!