You can prevent the installer from asking for an install location by running it with a command line argument pointing to the directory for the install. This can be done by making a copy of the 'Calibre Library' folder and all its contents. Portable media can occasionally fail so you should make periodic backups of your calibre library.
The installer will automatically update calibre Portable.
If you want to upgrade a previous version of calibre Portable, download the latest version of the installer from here and run it, choosing the location of your previous calibre Portable install. Previous releases of calibre are available here.
