To change the date and time on your event, go into Edit Mode by clicking on "Edit" on your event page. Then click on the date and time, and a calendar will open up. You can also change the timezone by clicking the default timezone at the bottom of the calendar.
Your guests won't be notified of any date or time changes - you'll need to send them a text blast if you'd like to notify them!
You can't change the date and time for events that are in the past. In these cases, we'd recommend duplicating your event page.