Click on the Events tab from the navigation. This will take you to your page with all of your events in it.
Find the event you want to edit and click on the "Edit Event" button.
You can edit any previous details within the event, or if you missed something out when creating the event, you can go in and add any missing details to the event.
Warning: once you have pressed this button, the event will be deleted and you cannot get it back. So be sure that you actually want to delete the event!