When you're updating your Drupal site, you'll need to put it into Maintenance Mode so that no problems occur for you or visitors.
Here's how to turn on Maintenance Mode in Drupal 7:
Go to "Configuration" then "Maintenance mode".
1) Make sure the "Put site into maintenance mode" box is checked
You'll now see a message saying "The configuration options have been saved".
You will be able to browse around your site without problems but visitors who aren't logged in will see the message above.
If you log yourself and can't get back in, just add /?q=user after your site address and you'll be back to the user login page.