How to change my maintenance page?
Hope this will help you,

Before site goes offline for maintenance mode, check the user who has permission to access the site during maintenance mode via admin/people/permissions
and check for this permission Use the site in maintenance mode. Default access will be given to the admin.

In Drupal 7, to create a custom maintenance page we have to override the default maintenance template file

We can do it by the following steps:


Copy maintenance-page.tpl.php from modules/system/
Paste it in active theme folder inside templates folder
ie., sites/all/themes/your_theme/templates/
Make the changes in new maintenance template file.
Clear the Drupal cache - configuration > performance > clear all caches


Note:


If you want how to Put Drupal 7 in Maintenance Mode, you can go through this link
To bring the site back online


The user who has permission to access the site during maintenance mode can
able to login using login page
Go to admin/config/development/maintenance and uncheck Put site into maintenance mode and save it.