This section contains articles that discuss the process of upgrading your Drupal installation (Drupal core and contributed modules) as a Drupal site owner.
If you are the maintainer of a contributed module, or if you have a custom module and need to port your code to be compatible with another version of Drupal core, see the updating modules section of the Developer Guide. For information on updating a theme, see Converting a Theme to a Later Drupal Version.
For problems during your site upgrade, please read the project page and, if necessary, post an issue there:
Drupal 4.7.x and earlier are no longer officially supported and should be upgraded to a supported version. If you are upgrading from 4.7.x to 6.x, it may be necessary to do an interim upgrade to 5.x before upgrading to 6.x
Upgrade instructions for upgrading to a particular version can also be found in the UPGRADE.txt file packaged with your Drupal files.
Also make sure you carefully take note of any special instructions on the page announcing the release of your version (e.g. http://drupal.org/drupal-6.2 for Drupal 6.2). You may also need to check the release pages of earlier versions if you are upgrading by more than one minor version (e.g. from 6.0 to 6.2).
The pages in this section of the site on the upgrade process (especially the sub-section "Introduction to upgrading") provide more detailed/background information on the upgrade process and instructions for some specific situations, and will make most sense if you first read UPGRADE.txt or review one of the following videocasts:
NOTE: One of the most IMPORTANT things to do BEFORE you upgrade, is to backup your site's files and database. See this example of an automated backup script, or Drupal's module Backup and Migrate for easy database backups.