The installation procedure on the Prestashop site is described here:
http://addons.prestashop.com/en/content/21-how-to
This documentation is old but corresponds perfectly to the current reality, all cases are precisely described and that’s why we decided not to re-write.
If the automatic installation process performed from the back office is not working, follow also the instructions in this documentation, the manual installation procedure is fully described.
The procedure for updating is identical. The update via the back office or via FTP does not overwrite your current settings.