1. When in Overview of your organiser account, you'll see a dashboard with your Next event and your Upcoming events.
2. Click the three dots next to the event you want to edit, and click Edit event.
3. You can also do this via the My events tab by clicking the same three dots.
4. You can navigate between the three pages - Event details, Ticket details, Select charity - depending on which elements of your event you'd like to edit.
N.B. Editing an event gives you access to exactly the same page as create event. There's some more detail on tailoring your event to your needs here.