You can change the time of an event by viewing it on the day view of the calendar, long-pressing and then dragging it up and down to adjust the time.

Alternatively, you can:

1. Head to the existing event you want to change the time or date of 
2. Tap on the date of the event
3. Choose the 'Change time' option 
4. Select the new time & date 
5. Tap 'Save'