How to update description and thumbnail of the Twitch stream videos via Twitch API

Correct.

Streams do not have a settable thumbnail or does a stream have a description.

VOD’s have no API support for modification of the VOD

I cannot speak to what Twitch plans or doesn’t plan to do.

Uservoice is the home for feature requests: https://twitch.uservoice.com/forums/310213-developers

A browser manually has to be used to give a VOD a custom thumbnail and/or description.

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