Why I didn't get xcode updates for xcode 9.0.1

I'm using Xcode 8.3.3 and my os version is 10.12.6. when I look for updates in App Store it shows if there's any availble updates. but doesn't show any update for Xcode. but if I searched for xcode then it shows the button Install, not for Update. I'm wondering why is that. what happen if I Install. does it install as a new Xcode or, update the existing verstion to 9.0.1 hope your answer with this.