The update has been signed the same as the app you have installed, so it will install without wiping the data you already have stored. If you just download an apk for the app, not for an update, it can't install, because the app is already installed, you need to uninstall the app first. If there's no stored data, that's fine, but if it's a game, or a browser and you have bookmarks, or it's some other app you have data for, you probably don't want to do that, unless the phone is rooted and you know how to back up that app's data first.