Simply go to the Events page and click the "Past Events" tab. The following page will show all of your past events sorted by most recent event at the top. Please keep in mind that you cannot edit the event times for an event that has passed.
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Update 01.06.2014 - Facebook is messing with things again and the "Past Events" tab/link is there, but it's a dead link, just text. If you want to view past events it's laid out like this: https://www.facebook.com/whateverthenameofthefacbookgrouporpersonis/events?key=past