How do I change the appointment time of an event in Calendar?

Tap on the event to bring up the information box. Tap on the time to edit it. Or, just drag the appointment to a different day and or time.

You may also want to take a look at the user guide:

Calendar User Guide for Mac - Apple Support

sapper wrote:

'Tap on'? Maybe on an iPhone, NOT on a MacBook. Thanks for the link.

Position the cursor and then tap on the mouse or trackpad.