It depends on the type of update.
Your app is automatically updated with your new content, as well as new colors and new design options.
The only things that would require new submissions are items related to the app itself:
- app icon,
- app startup image (splash screen),
- app name,
- app description,
- categories and keywords,
- and the mobile website URL (in settings>domain), if you change it after your app is published.
If you add new features in your app after it has been published, you don’t need to resummit again.
Only these features would require to submit the app again if you have not added them before publishing:
- Audio & Radio features (you can add them after your app is published but it will not work in background when the app is closed)
- Scan Code
- AdMob for iOS (For Android you can add AdMob banners or interstitals after your app is published, but iOS will need new publication)
But, adding or changing content to these features after your app has been published doesn’t require you to publish again.