This actually downloads the models, and is NOT just new SiriUI. Hence, this process is complex and probably not worth it.
- What does not work: Writing Tools, Memories, Reduce Interruptions, Image Eraser and other tools that are within official Apple Intelligence on supported devices.
- What does work: Slightly better Siri, New UI
- (e.g) You can ask about the iPhone/iPad User Guide, or ask to play a specific song in Spotify, which Old Siri could not do.