Looking at the home screens, the primary difference you'll find is that in iOS 26 the background of the dock and the search option that sits in between the dock and the home screen icons are more transparent and have a sheen to the edges, whereas in iOS 18, these are slightly darker.

Testing with developer and public beta versions went smoothly for even the oldest supported models. But there are safety measures if upgrading to the beta or release version goes awry.

Apple announced in June that the release of iOS 26 would first appear as developer betas as usual, with the first public beta set for July. When it reaches general release it will be on the 2025 releases, iPhone 17 series, as well the iPhone 16 series and plenty of much earlier models.