You do not need to do anything. We have it saved in our system.
You are able to change your RSVP. Follow the same steps for when you originally RSVP'd.
If you already booked your reservations, you do not need to change them to September 26th if you are still planning on attending. You should receive an email from the Marriott saying that they moved your reservations to the same day(s) of the week for that weekend in September. If you now need to cancel your reservation, you have until 48 hours prior to check-in to cancel for free.
Yes, we still plan on having a dinner and dance at the reception in September.
Here is the link to the live stream of the ceremony: https://www.youtube.com/watch?v=eKcE8yIW9WE the ceremony starts at 2pm