Change an event's dates with Change dates on its Overview. The new dates apply straight away, to the live version and every draft, and a published event is updated in the app.
Event days
Your plan includes a number of event days. They're counted in 24-hour spans from the event's start, so an event from Friday 6 PM to Sunday 11 PM uses 3.
To make the event longer than your event days allow, buy more event days with Buy more on the Overview, or upgrade to a larger plan with more of them.
Date changes
Moving the event's start by 12 hours or more uses one date change. Your plan includes a number of them. Changing only the end, or moving the start by less than 12 hours, doesn't use one.
If you've used your date changes, the Change dates dialog says so, and you can buy another with Buy more.
See Plans and add-ons for what each plan includes.