If it was the one-click, it would have created a folder called 'yourdomain.com.old' in your home folder. Rename the 1.5 installation to yourdomain.com.new, then rename the old one to yourdomain.com
Next, visit the Panel under Manage MySQL and do a Restore DB for your Joomla database. Pick a backup that's from before you did the one-click upgrade. They backup the database once a day. When you restore, pick the last option to overwrite your old tables.