Warning
This is patched as of iOS/iPadOS 18.1 DevBeta 5. If you want to follow this, stay on Beta 4.
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.