If you've already sold tickets or been paid out for a Splash event, you will not be able to change the date and time for that page.
If the date and time are locked in the Settings tab, you can create a brand new event page.
Check out: Can I create a recurring event?
Using one page per event is crucial to the operations of your ticket sales and event payouts. Make your accountants' lives easier by keeping accurate accounting information organized per event.