If you have organized your decks into a tree, whether intentionally or not, you may wish to eventually move a subdeck back to the top level. To do so, you can drag it to either the very top or the very bottom of the deck list. (Alternatively, you can rename it and remove everything before the deck's displayed name.)
More from the knowledge base
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Card Appearance
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Interface
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Anki Ecosystem
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Problems
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Collection Management