Every time a new beta is released, it’s generally accompanied with a new Xcode beta release. So if you’re programming against a beta OS, not only update your device with the latest OS but always plan on installing the latest Xcode beta, too.
Regarding whether code written for iOS 9 will work on iOS 10, if you have existing iOS 9 apps out on the store, they will generally work fine with iOS 10. But when you go to recompile your old iOS 9 code with iOS 10 (e.g. you’re planning on issuing a new release of your app), you may have to change your code to compile correctly.
Xcode 8 includes both Swift 2.3 (for which there will only be a few changes you’ll likely have to make) and Swift 3.0 (requiring even more changes). Xcode 8 has conversion tools to help automate the conversion from Swift 2.2 to 2.3 or 3.0, respectively. Needless to say, before converting your code from Swift 2.2 to 2.3/3.0, keep a back copy of your old Swift 2.2 code, because you don’t want to lose 2.2 code, because you can’t use 2.3/3.0 in production apps until iOS 10 is no longer beta software.
solved Will there be new Xcode releases for new iOS 10 betas? [closed]