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.
All done! Your event has been edited and the event will show up with the new details on the calendar.