Once confirmed, you cannot edit your event code.
If no one has redeemed your code, you can cancel your code by clicking the "cancel event code" link at the bottom of your confirmation email. This will push a full refund, cancel the code, and will allow you to go through the portal to create a new event code with the correct information.
Please note that you will not be able to create the same alphanumeric code as your previous one. It must be a different alphanumeric sequence.