If you're eager to check out the next version of iOS 10, you'll have a chance to do just that starting today. Apple is opening up its beta of iOS 10 to the public, letting anyone with an iPhone check it out (that likely means Apple Watch owners will be able to try out the beta of watchOS 3 as well).
As always, this is beta software and may be buggy. So you may want to consider installing this on an older phone or a secondary device like an iPad, rather than the main phone you carry with you each and every day.
macOS Sierra's beta is going public, too
iOS 10 includes major updates to Messages and the lock screen, as well as other design tweaks and smaller new features. You can read more about them here.