In version 1.9 we have update massive change for multishop compatible. If you do not use multishop.Do not update all files.
1. Update 1.8 or later to 1.9 single non multishop prestashop 1.5.5
i) overwrite the following files from theme folder.
and themes all the tpl file
N.B: Do not update themes folder
For this image slider will not work.
2. If you want to use with side bar on home page and the image hover features and other then you have to do flowing way .
In current version we have some improvement and hook add to show product on hove .
If you do not want to show product on hover So do not need to reset your admin panel.
Back up your site before update also please view this video http://www.screenr.com/RyDH
over write the full file.
If you change translation file , that do not overwirte your language file.
and overwrite that
N. B: For multishop work smartimageslider,smartmegamenu,smarthomeadverticement is build newly. So it may not work if you update. After update you need to reset those module. If you are using version older than 1.9