This site was using Joomla 1.5.11, prior to Jan 2013 and long overdue for an upgrade to Joomla 2.5.1...Should I upgrade?
First things first - all the blogs that I read say that before any uprade it is essential to be on the most up to date stable version of 1.5, which is 1.5.26...Hmm, need to get from 1.5.15 to 1.5.26
After doing a full backup - Full backup for me is running Akeeba and using ftp to download the backup file from the server to my home computer...What you say - don't have Akeeba...this should be installed immediately. I also copy all the root Joomla files to my computer and export the SQL database and save the file. Maybe slightly overkill, but too much backup cannot be a bad thing. See my article for manual backup of Joomla.
The easiest way to upgrade the 1.5.15 is to install the patch that is documented on the Joomla web site. Simply download the zip file, unzip and FTP up the files to your site.
Once you are at 1.5.25 then simply go to the back end administration to upgrade to 1.5.26. At this stage I went through the full backup process again before contemplating getting to 2.5?