You sure can! If someone changes their plans after already submitting their RSVP in your guest list, you can still change this in your Guest List!
To get started, pull up your guest list on a computer, go to your Track RSVPs page, and click on the guest you'd like to edit.
Once you've found the guest you'd like to edit click their name and you'll see the Edit Guest pop up box. Click into the RSVP Status tab and you can change their RSVP there!
Working from the app?
Tap into the Guests + RSVP icon on your home screen > then tap into the guest you want to edit > tap into the RSVPs tab and you can change any of their responses there!
