] is a link to code that purports to allow you to find an existing appointment. And here
] is a link to code that purports to allow you to create a new appointment.
I would think that once you're able to find the appointment you want to edit, you should be able to edit it using a technique similar to the "create" scenario. Please note that I simply googled "vb.net outlook find appointment" and "vb.net outlook" to find these, so I don't know whether they will work for you or not.