How to see wp-config file from WordPress dashboard?

You could create a plugin for a dashboard widget, zip, upload and activate it.

Example:

<?php # -*- coding: utf-8 -*-
/**
 * Plugin Name: Configuration Dashboard Widget
 * Description: Show the current installation path and the content of the <code>wp-config.php</code>.
 * Version:     16.10.15
 * Required:    4.0
 * Author:      Thomas Scholz
 * Author URI:  http://toscho.de
 * License:     MIT
 * License URI: http://www.opensource.org/licenses/mit-license.php
 */

add_action( 'wp_loaded', function() {

    current_user_can( 'update_core' ) && add_action(
        'wp_dashboard_setup',
        function() {
            wp_add_dashboard_widget(
                'config_widget',
                'Configuration',
                function() {

                    $wp_config = FALSE;
                    if ( is_readable( ABSPATH . 'wp-config.php' ) )
                        $wp_config = ABSPATH . 'wp-config.php';
                    elseif ( is_readable( dirname( ABSPATH ) . '/wp-config.php' ) )
                        $wp_config = dirname( ABSPATH ) . '/wp-config.php';

                    if ( $wp_config )
                        $code = esc_html( file_get_contents( $wp_config ) );
                    else
                        $code="wp-config.php not found";

                    print '<pre class="code" style="overflow: scroll;max-height: 20em"
                            >Installation path: ' . ABSPATH
                          . "\n\n"
                          . $code
                          . '</pre>';
                }
            );
        }
    );
});