To edit an event, login to your admin section, click the dropdown "Add-on Modules" menu in the top menu of your admin section and choose "Events Calendar".
This will bring up a page which displays the current month of the calendar with all events which have been setup. Click on the event you wish to edit. This will open the "Edit an Event" page. Editing an event is just like adding an event. For more information about using the event editor features, please see the help page on Adding an Event. Make whatever changes you wish to make and then at the bottom of the page, click the "Save Changes" button.