Much more than a simple cell phone, the iPhone allows you to receive e-mails and manage your calendars (not to mention play games, music and videos and take pictures). With the iPhone's "Calendar" application, you add, edit and manage your appointments. If you wish to completely reset your calendar, you must delete it (thus deleting all appointments) first.
Open the "Settings" application on your iPhone.
Select "Mail, Contacts, Calendars."
Select the account that contains the calendar you wish to reset.
Move the "Calendar" slider to "Off." A pop-up message will ask if you want to delete the calendar and all its data from your phone. Click "Delete from my iPhone." This will completely erase the calendar and all appointments.
Move the "Calendar" slider back to "On" to start fresh.