Normally, you'd download Xcode from the Mac App Store and let it (auto)update when a new version is available. There's a better way though. Don't download from the app store. Instead, download from Apple's Downloads for Apple Developers page. This lets you easily run multiple versions of Xcode in parallel (you can rename the .app file) and avoids the issue where the Mac App Store decides to start a download/install process and you are stuck not having Xcode available.