Below we are going to show you how you can upgrade your Joomla 1.0.x to the stable Joomla 1.5 version.
We would like to note that it is highly recommended to backup your existing Joomla web site before you proceed with the migration process.
Joomla 1.0.x to 1.5 migration requires a little bit more tweaking than the normal Joomla upgrades.
Step 1: Install the Migrator component
The first step we should follow is to install the Migrator component to our existing Joomla web site. You can download the Migrator component here.
For our tutorial we are going to use the migrator_rc6.tgz archive. Left-click on the migrator_rc6.tgz link and save the archive on your PC. The archive itself consits of a Joomla component, which is actually taking care of the migration process. Fortunately installing Joomla components is not a big deal. Go to your Joomla administrative control panel (http://www.your-domain.com/administrator) and log in:
Once you are in from the top Joomla navigation menu mouse over the Installers tool and select Components from the drop down menu:

Click on [Upload Package File] and locate the migrator_rc6.tgz from your hard disk. Confirm the installation by clicking on the [Upload file & install] button:

Press [Continue..] at the Upload Component – Success screen. Once the component is installed mouse over the Components tool available at the top Joomla menu and select Migrator:

The component menu allows you to migrate your current Joomla 1.0.x data to the Joomla 1.5 database format.
Our tutorial covers only the steps to migrate a base Joomla web site without any third party extensions and components installed. However, if you use any such, make sure to imput their migration scripts by using the 3rd Party Information and Add 3rd Party Migrators options offered by the Migrator component:

Any additional components / modules/ mambots should have migration scripts provided by their respective developers on the offical project sites.
Step 2: Create Migration SQL File
To proceed with the database migration select the last link available near to the bottom of the Migrator component page – Create Migration SQL File.
If the things go smoothly and there are no errors reported during the SQL data export process, you will see the following screen:

The next step is to download the extracted SQL from your 1.0.x Joomla:

You can now download your SQL dump by clicking on the Download link available on the right section of the page and upload it into your Joomla! 1.5 installation. Later, when importing this SQL file into your 1.5 installation, make sure to select Load Migration Script and use the prefix ‘jos_’ (by default).
Step 3: Perform a clean Joomla 1.5 installation
Next we will perform a clean Joomla 1.5 installation. First obtain the latest installation package from:
http://joomlacode.org/gf/project/joomla/frs/?action=FrsReleaseBrowse&frs_package_id=2892
Select Joomla-1.5.0.zip from the file list and download it to your Desktop. Next log in cPanel and go to File manager. Once you open it, you would need to create a new directory called dev (since we are using it for development) inside your public_html directory:

Change your current working directory to the public_html/dev/ one. Click on the [Upload] icon available at the top File manager navigation menu, select the Joomla-1.5.0.zip archive from your Desktop and it will be automatically uploaded:

Go back to the File manager, select the Joomla-1.5.0.zip installation archive by clicking on its name and hit the [Extract] button available near to the top right corner of the File manager:

It will ask you for the directory where the archive should be extracted. Just leave it to the default value and confirm.
Another window will open which will show you the log from the extract porcess and if there are no errors reported you are ready to proceed with the next step.
Open the dev folder by visiting http://www.your-website.com/dev. You will see the Joomla start installation screen. Select your language, you should already have a database and user to fill them in.
Proceed with the installation further untill you reach Step 6: Configuration:

Fill the name of your web site, your admin e-mail and password details.
The key part here is to choose the Load Migration Script option:

Fill the old table prefix _jos (unless your previous Joomla has used a different prefix) and leave the encoding unchanged unless needed and you are familiar with this option.
By clicking on the [Browse] button you have to select the previous Joomla 1.0.x migration dump from your Desktop.
Make sure to select the This script is a Joomla! 1.0 migration script. check box as well.
Next confirm the progress by clicking on the [Upload and execute] button.
Once the data is imported you should see the following screen:

Click on the [Next] button near to the upper right corner.
You need to completely remove the installation directory by using the File manager available in cPanel.

Now when you open the front end of your newly installed Joomla 1.5 you will see all the articles and data from the previous Joomla imported.