Before uninstalling, please verify your data is synced by checking the Web App. See here: How do I know if my data is synced?
Uninstalling:
If possible, save a copy of the encryption key from Day One > Settings > Sync > Encryption Key if not already saved. Or use another device to locate and save the key.
Close Day One.
Tap and hold on the Day One icon.
Tap Remove App
Tap Delete App
