If you're running Android, you want to use the Chromecast app, and select 'Devices'. Then, choose the menu button (the 'kebab' button, which is three dots above one another), and choose 'Device settings'. The firmware settings are at the bottom of the page.
As an aside: you should also test Google Play Music, which both allows you to play from any song in its catalog (just like Spotify), but also lets you upload your own tracks to the system, so all your music is in one place. Those uploads are either matched with existing songs, or are, I believe, transcoded.
