Knowledgebase
How to upgrade Joomla from 1.6 to 2.5 using the automatic updater
First you have to make sure that your Joomla is at least version 1.6.5 or 1.6.6. If your Joomla 1.6 is an older version, you have to upgrade it to 1.6.5 before you can use the automatic updater to upgrade it to 2.5. To upgrade older version of Joomla 1.6 to 1.6.5 you need to download the patch package 1.6.0 to 1.6.5 from the official Joomla site. Then unzip the archive on your local computer and use an FTP client to upload the files and folders inside it to the root Joomla directory on your Joomla hosting account.
To update Joomla 1.6.5/1.6.6 to 2.5 log in to the backend of your Joomla and go to the Extension Manager. Click on the Update tab, then on the Purge Cache button and on the Find Updates button; both buttons are in the upper right corner of the Update tab page. The last 2.5 version will be displayed on the page. Mark the checkbox in front of it and click on the Update button in the upper right corner. Joomla will first be updated automatically to version 1.7. Once this is done you'll be prompted to mark the checkbox for the second update package. Do it and click again on the Update button. After a few minutes Joomla will be updated to 2.5.
Next, you also have to update the database. Click on the Database tab of the Extension Manager and then on the Fix button in the upper right corner.
You should also make sure that all the extensions were installed during the update process. To do this click on the Discover tab of the Extension Manager and then on the Discover button in the upper right corner. If there are any extensions listed on the page, install them.
For more details on the update process and on some of the errors and problems that may occur read the tutorial on how to update Joomla from version 1.6 to 2.5 with the automatic updater.