First, tap the Manage Guests
button to pull up a list of all your invitees.
This will show your guest list with each guest's name, email, and RSVP status. Tap on the guest you want to edit to see their information.
On the next screen, you'll see the guest information including their RSVP status along with the dates they were sent and viewed the invite. Click the Edit
option in upper-righthand corner and you'll be able to change your guest's RSVP (you also have the option to change the guest's name as well).