Update
As Sam commented this behavior seems to have changed recently (maybe with iOS 11 App Store Changes?) so it is no longer possible to update the App Description without an App Update. The Description field is now “locked” for apps that are in Ready For Sale state. Other metadata fields continue to be editable as specified in the official documentation linked below.
Original
As RJH mentioned in a comment, you CAN actually update the description (as well as the Marketing and Support URL) for an app version already in the app store.
Here’s the link again with direction to the specific section:
https://help.apple.com/app-store-connect/#/devfc3066644
In order to do this, from within your App in iTunes Connect you must:
- Select the specific app version (left sidebar)
- Update the description field
- Don’t forget to click Save!
Apple doco says your changes are posted immediately and should be reflected in the App Store within 24 hours.