Before starting this tutorial we would like to inform you that Plugins, Modules, Templates etc. that are not compatible with Joomla 2.5 will stop working. That is why we recommend you to first double check if they are compatible or if their developers have released newer versions!
Upgrading your Joomla 1.5 can easily be done using the jUpgrade plugin which is provided for free by its developers. All you need to do is open a free account on their website:
http://redcomponent.com/redcomponent/jupgrade
*Note that the plugin is compatible with the latest Joomla 1.5 version.
Once you have the plugin downloaded, you must install it on your Joomla. You can follow this tutorial on how to install Plugins onto your Joomla.
Even though, the tutorial is for Joomla 2.5 and 3.0, the method is the same for Joomla 1.5 as well.
Once you have the Plugin installed, you need to navigate to your Joomla Plugins page Manager located under the Extensions tab -> Plug-in Manager. Look for the System – Mootools Upgrade and enable it.
After that proceed to the Components tab -> jUpgrade.
Then you will see a big UPGRADE button to proceed with the upgrade.
We would like to mention that this will create a copy of your Joomla website automatically in a subfolder called jUpgrade and upgrade your Joomla to the newest available version. You can double check your Joomla website in this subfolder and see what is working and what is not in order to make final changes.
The last step will be to move your Joomla website from the subfolder to your hosting account main folder. To do so, you can follow this tutorial.