Upgrading Joomla 1.5 is a standard procedure to apply patches and security fixes.
To ugrade to the most recent version,
1) Backup the original fiules and database.
2) Download the appropriate upgrade version from joomlacode.org.
For example, if you use Joomla 1.5.10 you would download:
3) Unzip the package on your pc.
4) Use FTP or Cpanel ‘File Manager’ to transfer the updated files to the server. Make sure you do not remove any components or installed extensions. To keep this from happening, always overwrite files but do not delete files.
5) Login to the administrator backend and you will see the new version.