Ahh I’ve actually answered this question but it was a long time ago…
From the main screen, click “Browse” and then on the left select the deck you want to take cards from. Select the cards you want to move (or click on one and then ctrl + A to select all of them), then click “Change Deck” and select the destination deck, then click “Move Cards”. You can delete the deck after, since it will be empty now. All of your study data moves with the cards too, fyi.
Good luck!
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https://www.youtube.com/watch?v=qLQzB-1u-dg