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]. So 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, but you should pay very close attention to the details of an upgrade. Upgrades would be defined as a major.minor(2.5) to a different major.minor(3.1) version or a major.minor(3.0) to a major.new minor(3.1).
Updates are an easy update to the next maintenance release, for example version 2.5.13 to version 2.5.14. This would be a maintenance release of the 2.5 version.
Upgrading from a major.minor to major.minor version can be done with the built-in core Upgrade Component. However, before upgrading you will have to do some homework. You will need to gather information which will include the following:
Above all, realise without the correct preparation and knowledge, you may break your 'live' site.
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.
A lot of Joomla! CMS version 2.5 was based off of the 1.5 version, but at the same time there was a massive effort put into advancing Joomla CMS version 2.5 to where it is today. This effort required many core code and database changes. These changes from the Joomla! CMS version 1.5 to version 2.5 were so large and sweeping, there is no core upgrade path. If you are using a Joomla! CMS version 1.5 this will requires a migration to upgrade.