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 web app at https://dayone.me/settings. Sign in with the desired option (Day One account, Sign In with Apple, Sign in with Google).
Once signed in, Click Settings under the list of journals on the left-side. 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.me, 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.