Thanks for reaching out about this in the Community. We're happy to shed some light on the situation.
If you're missing the option to change the cover of your playlists, you might be on a new desktop experience we're currently testing. For now, it's not possible to edit playlist covers or descriptions with the new test version.
We understand this is not ideal, so if you'd like to be able to switch between the test version and the old design, we suggest you add your +VOTE to this idea - it's the best way to provide your feedback for new features such as this one.
Hope this info is useful. We'll be here if you need help with anything else.
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