If you ever have the need to delete your journal data from Day One’s servers, follow these steps:
Sign out of Day One Sync on your device.
Go to https://dayone.me/settings in your web browser and log in.
Scroll down and click to delete data from the servers.
Doing this will remove the data from Day One’s servers. The data on your local devices is unaffected.

Please note there isn’t a way for you to undo this action, so be sure you truly want this data removed before proceeding