we started to develope an application with swift for iOS.
when we started it latest version was developer target 9.3.
now there is developer target 10 available. Can I install application that created with developer target 9.3 on iOS 10 ?
['https://Stackoverflow.com/questions/40625869', 'https://Stackoverflow.com', 'https://Stackoverflow.com/users/4433550/']
Yes, You can install lower developer target on higher iOS version.
**Deployment Target** is the **minimum required iOS version you application needs to run**.

You can build an application with SDK 10 that runs under iOS 9. But then you have to take care to not use any function or method that is not available on iOS 9. 

Also Always check to see if you are using deprecated APIs; though still available, deprecated APIs are not guaranteed to be available in the future.