WordPress: Ajax (POST) to update post meta key

functions.php

/**
 * Update post meta key via ajax POST request
 *
 * @see http://wpquestions.com/question/showLoggedIn/id/10604
 *
 * On the remote site send a POST request to:
 *
 *     http://example.tld/wp-admin/admin-ajax.php?action=update_post_meta
 *
 * with the following payload:
 *
 *     security, post_id, meta_key, meta_value
 *
 * TODO: 
 *     - Consider IP restriction.
 *     - Remember to update the security key below!
 *
 */

add_action( 'wp_ajax_update_post_meta',        'wpq_action_handler' );
add_action( 'wp_ajax_nopriv_update_post_meta', 'wpq_action_handler' );

function wpq_action_handler() {

    // User input:
    $args = array(
        'security'   => FILTER_SANITIZE_STRING,
        'post_id'    => FILTER_SANITIZE_NUMBER_INT,
        'meta_key'   => FILTER_SANITIZE_STRING,
        'meta_value' => FILTER_SANITIZE_STRING,
    );
    $input = filter_input_array( INPUT_POST, $args );

    // Init:
    $security = '123abc'; // Edit this!        
    $success  = 0;

    // Update meta key for a given post	ID:
    if( $security === $input['security'] ) {
        $post = get_post( $input['post_id'] );
        if( ! is_null( $post ) ) {
            if( get_post_meta( $post->ID, $input['meta_key'], true ) ) {
                if( update_post_meta( $post->ID, $input['meta_key'], $input['meta_value'] ) ) {
                    $success = 1;
                }
            }
	}
     }

     // Output:
     if( $success ) {
         wp_send_json_success();
     } else {
         wp_send_json_error();
     }
}

How to use this function in the page ?