Click or tap the privacy symbol in the top of the window while you're in edit mode, and choose the setting you want. You can also adjust the settings anytime from the Share and Export menus. Look for the lock icon highlighted in the image below.
If you change a deck from Public or Restricted to Private after a link to that deck has been shared, anyone clicking that link will get an 500 error message as if the deck never existed. Only the author can see a private deck when signed into their Haiku Deck account.