If you're not concerned about saving the old event in your calendar, what I recommend is changing the date and time on the calendar even you have. In essence, you're just moving the date forward. The "old" event will be gone, but you'll have the new event.
If you want to keep the old event, here's the only way I've found that let's you "copy" an event (I got this from the Apple support forums):
"Here’s how I duplicate a Calendar event on my iPad Air or my iPhone 5. (iOS 7.1)
Edit the event you want to copy.
Choose Repeat, Every Day.
Change the End Repeat to be the next day. (Watch out, it defaults to a week from today.)
When you press Done, you will have duplicated the event the next day at the same time, but you are not finished. These two events are linked, and you have to unlink them.
Edit the second event you just created and change it to Repeat: Never. You can also move it to the date and time you want.
When you press Done, respond Save for future events.
Edit the first event again and also change it to Repeat: Never, and save for future events. Don’t worry, the second event has been unlinked and it will not be affected.
Now, move the second event to the date and time where you want it. You can add new notes or whatever. It is an independent event now.
Warning: If you don’t unlink the events so both are Repeat:Never, Calendar can get confused and not allow any further Repeats to be made to either event."
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Hopefully, one of these two will be helpful (as I can't find a specific copy/paste function in Apple's calendar...
Marilyn
