Menus are a collection of links (menu items) used to navigate a website. The Menu module provides an interface to control and customize the powerful menu system that comes with Drupal. Menus are primarily displayed as a hierarchical list of links using Drupal's highly flexible blocks feature.
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Drupal 7 has a feature to display a maintenance mode page when you are working on your website. Users can see only maintenance message “Use the site in maintenance mode” when maintenance mode is enabled. Admin can access entire Website.
How to update Drupal core with a newer version of Drupal of the same category or same base version