As per subject… I would like to change a release group cover, didn’t find the way to achieve this… 
F.
If there are multiple releases with cover art in a release group, you can select which is used as the release group cover with the link “Set cover art” in the sidebar under “Editing”. If only one release in the group has cover art, that cover art will automatically be set and if none of the releases have art, you’ll have to add cover art to one of the releases first.
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Further to what @mfmeulenbelt said, if a release has more than one image marked as “front”, I believe that only the first one will be available to select as the release group cover image.
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And also generally there is no problem in letting the default dynamic automatic release group cover being chosen from its evolving content (releases). 
But sometimes a non-representative edition can be chosen and you want to force another one.
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