When you want to update to a new version, or go back to a previous version, you can do this from the Boards Manager found in the Arduino IDE select Menu | Tools | Board: “…“ | Boards Manager … Then type “rfz” next to the Type ALL to narrow the search result and click on “RFzero SAMD Boards (GPS based RF Clock Generator) …” and select the version you want to use.
If you want you can also let the Arduino IDE automatically notify you when there are new versions, of any library, available. You do this in the Preferences from the Arduino IDE select Menu | File | Preferences | Check for updates on startup.
Please note that since the RFzero is both a hardware board and software library you cannot find the RFzero in the Arduino Library Manager. Neither are you able to find the RFzero library in the default Arduino “library” subdirectory of the Sketchbook location nor should you copy/move the RFzero library to this location.