In the main full-page UI, not the Sidebar, we can't select the model. At least I didn't find a button for this anywhere though it's obvious in the Sidebar UI. The model marker (v3.5) is not a selector. Plus, on the Sidebar, it's already 4o-mini (in my case) so it doesn't even make sense why it is v3.5 in the main full-page UI. The new UI is cool btw. :)