ImpressPages can be updated by using System admin panel.
Also, you can update ImpressPages manually, by replacing the content of Ip directory with Ip directory from the downloaded archive.
If you have updated your files manually, then you must update your database by pointing web browser to http://www.example.com/?pa=Update URL.
ImpressPages does the copy of all your files to file/tmp directory. So in case you don't like new version, you can roll back.
But keep in mind, that you should have done the database backup and restore previous version of the database too.