Submit iOS build update without re-uploading screen-shots and app-previews

Question

I want to submit an update for my iOS app, but this seems to require submitting a new version which entails re-entering all information, screenshots, videos, etc.

I can\

Answer 1

There is no need to upload screenshots, videos and other information again and again when you are publishing a new version. Unless you have a new functionality which you want to be presented in the screenshots or you have to support a new apple device. For example your previous version didn’t support iPhone X, then when you create a new version, there won’t be any screenshot for iPhone X. So you need to upload screenshots only for that specific device.
So, just create the New version and upload the new build and add - What's new in this version. You are then good to go.
As to your question on whether it is possible to upload a new build to existing live version. Answer to that is - NO. Once an app is live, you always have to create a new version on iTunesConnect and upload build configured with that version number.
As an example, here are two images :

1. Already existing app on iTunesConnect (live version)

2. New version created. Screenshots already there by default.

Answer 2

For anyone googling here:

The problem is this:

It annoyingly defaults to (say) 6.5 !!!

(Whatever is current as you are reading this.)

And it's likely you have no images you likely have no images for that size!

Simply click on the other sizes to see your existing images!

That's all it is.

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2019 note ..

As @phihag mentions below, they're always changing "just which one is actually required," but in all events you can click around away from the default to see what's happening. You may then come to realize that Apple now requires image sizes that are NOT in your previous release, and so you now need to generate a new set of images.