The iPhone installs updates to the iOS operating system through the iTunes sync process. During this sync process, the Calendar application on your iPhone may also sync with the calendar application on your computer. Depending on your iTunes settings, this may result in the deletion of calendar events from your iPhone.
Changing iTunes Settings for Future Syncs
Connect your iPhone to your computer using the data cable. Open the iTunes application and select your iPhone from the list of "Devices" in iTunes. Click the "Info" tab. If you do not want your iPhone to delete calendar entries during the update process, select the "Sync Calendars" option. Scroll-down and uncheck the "Calendars" option under the "Advanced" subsection. During future updates, your iPhone will not delete calendar entries.
Recovering Missing Calendar Entries
If you created a backup of your iPhone during the update process, you can recover your missing calendar entries. If you did not create a new backup, you cannot recover the deleted calendar entries. Click the "Summary" tab in iTunes and click the "Restore" button. Select the backup created during the update and click the "Confirm" button. Your iPhone will restore your calendar entries. The restore process will also remove the update from your IPhone, so you will need to update your iPhone a second time after restoring your calendar events.
Update your iPhone Again
After restoring the calendar entries, you will need to update your iPhone. Before updating your device a second time, make sure you changed the settings in iTunes to prevent the deletion of calendar events. Click the "Update" button and click "Confirm." Your iPhone will automatically download and reinstall the updates to the iOS operating system. Your iPhone will not delete the calendar events during this update.
Other Information and Updates
Your iPhone may also delete information about your contacts, mail accounts, bookmarks and notes during the update or sync process. To prevent your iPhone from deleting any information during the update, click the "Info" tab in itunes. Scroll-down and uncheck the "Contacts," "Mail Accounts," "Bookmarks" and "Notes" options under the "Advanced" subsection. This will prevent your iPhone from replacing information on your device with information on your computer during the update process.