As James said, in (under) 15 minutes it was done for me.
As always with these updates: first reboot your Rossini by power cycling it by the back switch, and wait some minutes before starting the upgrade. When finished, it shows Please Restart, and then reboot again by the back switch.
Until the Mosaic update is available: you can engage Expanse by pressing the Menu button on the front → Press input > Audio Settings → Press Menu → Press Input >>> 3 times → Press Menu to togglle Crosstalk - E1 - E2 - Off.
Edit:
Expanse E1 and E2 are both crossfeed processing algorithms that use different processing filters. Just try which setting suits you best for different music.