From your main dashboard pick the 'events details' tile. 

You can click to add an event and add details as needed.

Each event you have has an 'edit event' option at the end of the general information. Please be aware if you have already received RSVPs from guests for this event and you change the details guests may not notice the updated information.

You can find more information about how the event coordinator works here - https://gettingmarried.co.uk/event-coordinator-user-guide/