After you have published an App in the Apple App Store and if you wish to change the App description, you have to login to iTunesconnect.
When you have logged in, you will see the below screen. Click on ‘My Apps’ and select the App which you wish to update the App description.
You will then reach the following screen. If you select App Information as shown below. Most of the information on this page cannot be edited. If you wish to change the App’s name or category, you have to create a new App version.
To edit the App description, you have to click on the ‘Ready for Sale’ as shown below. Then the App description page will appear for you to edit. You can toggle between ‘iPhone’ or ‘iPad’ to edit the description.
Once you have your App published in the App Store, you are unable to change the screenshots. This is Apple’s announcement on 9 Jan 2013:
Beginning January 9 , app screenshots will be locked in iTunes Connect once your app has been approved. New screenshots may be uploaded when you submit a binary for an update to an existing app or a new app.
If you App has not been published, you can click on the ‘Prepared for Submission’ , and choose ‘Media Manager’ as shown below to remove and add new screenshots: