A new beta build is available via the builtin auto updater.
Make sure you have enabled the beta opt-in. Settings->misc->beta build opt-in
(its not available on the website because i want to test the builtin updater)
If the update fails to install open %appdata%/Spotify in your file explorer and select:
then right click and select properties, and un-check the read-only checkbox, then apply.
You can then try the update again.