How to Migrate to Joomla in 15 Mins How to Upgrade Joomla 1.5 to 2.5 - Page 3
How to Prepare for Joomla Upgrade?
1. Back up your Joomla 1.5 website
2. Install Joomla 2.5
3. Find FTP access details (for both Joomla sites)
Find more info in this article