I want to submit a update for my existing app in app store using xcode 6.3..with iOS 9 release is it mandatory to adapt ATS for apps submitted using xcode 6.3 or any xcode version & will the app update be accepted as it is build using xcode 6.3 while xcode7 has been released?
ATS is specific to iOS 9 and if you use iOS9 it is required you either make sure all your web resources are SSL or you disable ATS. Further, you cannot use iOS 9 with anything below Xcode 7.
The latest Xcode is here and you will notice that Xcode 7 does not have tvOS while 7.1 does. Apple ships specific SDKs with specific Xcode versions. Apologies if you knew all this already.
Therefore, if the question is more about adopting the latest SDK then no you don't have to, but within reason. Apple would like everyone to use the latest SDK but it isn't always feasible. I've seen apps in the App Store be updated without latest SDK support.
More information from Apple is here
Any Other SDK is Temporary
It is not practical for Xcode to force every project to build with the latest SDK. For example, it may be necessary to ship an immediate hot-fix version of app - without waiting until the app has been fully updated for a newer SDK.
But it's important to understand that using an older SDK is a temporary workaround, not a solution. Issues preventing your app from using the latest SDK should be fixed as soon as possible. It will be much easier to fix these issues while the older build system is still supported.
Important: No Legacy SDK version will be supported indefinitely