Updating Joomla 1.5.x
The method for updating Joomla series 1.5.x needs an ftp client or can be done with the file manager in your Host's Control panel. You can choose to use a Patch or the full installation zip/tar.bz2/tar.gz to update. Our preference is to update Joomla using the full installation package and omit the installation folder when uploading the files.
Make a backup of your site.
Download the correct file for your Joomla
Joomla_1.5.26-Stable-Full_Package
Unzip the files and ftp all except the /installation folder to the server. The method to Update Joomla is exactly the same as the one used to Replace corrupt files.