You can create an event by hovering over the "Explore" tab at the very top of either the homepage or your profile page, click on "Events" and then +Create in the top right corner below your profile picture. You'll be forwarded to a form where you can add the dates and details of your event(s).
If you've already created an event and want to change it:
Go to the Events overview then click on "My Places" (next to "Events"). This tab also includes your events.
To change an event, click on the pen and paper icon to the right of the event name, which will take you to the overview page.
You can also copy your event by clicking on the icon below the "edit" action button.