This can be a little confusing. Updating properties of an extension, including description, screenshots, and icon requires a version of that extension to be reviewed/approved and then released. Let me try and say this in a different way, that is hopefully simpler: description, screenshots, and icon should be considered a part of a version of an extension. They are updated when a version is updated.
Expanding upon what @mitch3b said: when you move from local test to remote test all properties associated with the extension at that point in time are put in our CDN. If there is an active version of an extension, updated properties are only applied when a new/update version is released. If you edit properties after submitting an extension for review, another version (or pulling the current version back into local test) is a required action.
Please let me know whether this makes sense!