This tutorial will show you how to put the Joomla website in maintenance mode.
1) In your admin panel, go to Site => Global Configuration
2) Site Offline – Select whether access to the Site frontend is available If Yes, the frontend will display or not a message depending on the settings under "Offline message" option.
Offline message – Display or not a frontend message when the site is offline.
3) After you save changes your website should be set under maintenance mode.
Feel free to check the detailed video tutorial below: