Just install it. It will detect your existing subsonic settings folder and leave it alone.
On windows you may have to reset the username password that the service account users, if applicable, and reset the amount of RAM to dedicate to Subsonic, but that's about it.
On a normal update, it's enough to switch your subsonic.war file. On special occasions, you need to switch your subsonic-booter-with-dependencies.jar too. Upgrades are normally built so that previous settings are kept. (But it's not a bad idea to make backups anyway - the default Subsonic database is known to sometimes crash, and then it's good to have your users etc stored somewhere safe)