Before you begin, it is essential you know what version of Joomla! you are using. This is especially true if you are changing a major version. If you are already confused or don't know what a major version is, you should read more about the release and support cycle of Joomla CMS Versions. Joomla is released with the following version designations, [major].[minor].[maintenance]. For example, version 3.1.5 is Joomla major version 3, minor 1, and maintenance release 5.
Since version 1.7, the updating and upgrading of Joomla are built into the administration interface as a component. The terms updating and upgrading might be used interchangeably. In many ways, updating and upgrading are alike since Joomla 1.7, but you will need to pay very close attention to the details. To help define them better, it is recommended you use the term migration which would be defined as a major.minor(2.5) to a different major.minor(3.1) version.
Updates are an easy update to the next maintenance release and are backwards compatible, eg version 2.5.13 to version 2.5.14 or the version 1.7.x to 2.5.28. For Joomla CMS 3, these updates span all minor and maintenance releases. Some examples are version 3.1.2 to 3.3.6 and 3.3.1 to 3.3.6. Please see the specific pages to review version from and to.
The documentation below is a funnel starting with Why Migrate. Simply follow the buttons at the bottom of each page that are applicable to you. The Self Assessment will help you determine if doing the migration is within your skill set. Please don't skip the Planning pages as it includes important information for your migration.
Documentation on migration of a legacy version to another legacy version, eg 1.0 to 1.5 or 1.5 to 2.5.
Use the links below for legacy documentation for older versions of Joomla! or lesser used versions of Joomla! You are encouraged to search the Joomla! Community forum for even more information and education prior to starting your upgrades/migrations.
Upgrading 1.6.5, 1.6.6, 1.7.x, 2.5.x to 3.x is covered by the following page.
Upgrading 1.6.5, 1.6.6, 1.7.x, to 2.5 is covered by the following page.
You must upgrade to Version 1.7 first if you are on version of 1.6 below the maintenance release of 1.6.5.