When you are viewing a specific video details the cards and end screen buttons are on the right side under "visibility" and "playlist"pic.twitter.com/jHYPqg10j4
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You just need to replace the word "cards" with "endscreen" in the previous link. Also, if you need to temporarily revert back to classic creator studio to adjust multiple cards/endscreens just remove the "&ar=2&nv=1" after the vidID from the links. That should do the trick.
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