You can delete journals you no longer need. Keep in mind that any journal you delete will also delete all the entries in that journal, so proceed with caution.
iOS
Tap Settings (gear icon in the top-right corner of the screen).
Tap Journals.
Tap the journal you wish to delete.
Tap Delete Journal, then tap the Delete Journal button that appears.
Tap Delete Journal again. (Yes, we really want to make sure that you don’t accidentally delete a journal. 🙂 )

Android
Tap Settings (In the sidebar or three-dots button in the upper-right).
Tap Journals.
Tap the journal you wish to delete.
Tap Delete Journal, then tap the Delete button that appears.

Mac
Go to Day One > Settings (in the file menu).
Click Journals.
Select the journal you wish to delete.
Click the minus button.
Click Delete Journal.
