If you have permissions to create/edit a calendar appointment, tap or click to open the calendar activity. Once open, you will see a purple pencil icon in the upper right corner. Tap/click the purple pencil (edit).
Screenshot below is from the mobile application:
You will now see a red trash can icon in-place of the purple pencil icon. Tap/click the red trash can icon to delete the calendar activity.
A confirmation dialogue box will appear as a dual-verification for deletion: