Day One accounts are used for syncing data to Day One Sync* and for other features like Book Printing, Instagram integration*, etc. Account details can be managed on the Server Dashboard at https://dayone.app/settings. Sign in with the desired option (Day One account, Sign In with Apple, Sign in with Google).
Once signed in, there are options to edit the email address, change the password, and manage linked accounts for Apple and Google. If a subscription to Premium was purchased via dayone.app, it will be managed here as well.
*Some features require Plus or Premium account statuses.
Instagram integration can also be managed here.
Options to delete the account or remove all sync data from Day One Sync are available here. This is a permanent action.
If signed into the Home view of the Server Dashboard, click the name/email/user ID at the top right, then Settings to view the settings menu.