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.

Changing Email Address
Adding or changing your Day One account email address can only be done at https://dayone.me/settings.
If you created your Day One account with Sign in with Apple, you could add an email address on this page to sign in to your Day One account with an email address and password. Please refer to this article for detailed steps.
If you want to change your Day One account email address, tap the Edit button on the Email section and enter your new email address.
A confirmation email will be sent to your old and new email addresses from support@dayoneapp.com. Please look for the email with the subject Confirm Day One Email Change
in the inbox of your new email address, click the Confirm change button to confirm the change.
If you do not see the email in the inbox, please try searching for it in the spam/junk folders. Send us a support request from the app if you have trouble receiving the confirmation email, and we’ll assist you from there.