Running Drupal Update.php

By Lynn Beighley and Seamus Bellamy from Drupal For Dummies, 2nd Edition

Any time you upgrade a module — or (for that matter) update your Drupal software — you need to run a program called update.php. Before you do so, however, make sure you’re ready to upgrade: Make sure you’ve done these three tasks:

Back up your database. Ensure you have a current and accessible database backup.
Back up your code. For this step, you make a copy of your entire Drupal directory on your web server.

You can copy the code using a file manager application on the control panel at your web host's site. Or you can use an FTP program. The basic idea is to copy all of the Drupal code somewhere safe. Your best bet is to make a copy on your local computer that you can easily delete after you’ve confirmed that the update ran as expected.
Put your site in maintenance mode. Choose Configuration→Development→Maintenance mode and select the Put Site into Maintenance Mode check box. Click Save Configuration.

When you’re ready, updating is a pretty straightforward process: You log in as administrator and browse to the update.php file on your website. (It will be located at http://yourwebsitename.com/update.php. For example, ours is at http://drupalfordummies.com/update.php.) You’ll see a page listing the steps you need to follow next.

Now you’re ready to run update.php. Follow these steps:

Browse to your site's update.php. Click Continue.

This screen shows you any available updates for the current version of Drupal, modules, or themes you’ve installed.
Click Update.

Any updates you chose on the last screen will be made.
Choose Configuration→Development→Maintenance mode and deselect the Put Site into Maintenance Mode check box.
Click Save Configuration.