There are many ways to get Joomla! 1.5:
- You can download it from Main site of Joomla.org - https://www.joomla.org/ (Beta version, RC version, Stable version)
- If you want to download the latest development version, you can download the Nightly Builds version - https://dev.joomla.org/content/view/17/60/
- If you have a SVN software, you can download last SVN version. All information the about last SVN version you get here - https://dev.joomla.org/content/view/17/60/
After you download the Joomla! 1.5 package, you can upload it to your server - How to upload your files to your server via FTP.
Go to your Joomla! 1.5 site and follow the Installation instructions:
- All information about Installation Guide you get here: https://help.joomla.org/content/category/15/99/132/