Yes! You can easily do this right from your Mitzvites.com account.  Once you are logged in, click on the "Pie Chart" icon and then click on "RSVP Statistics" on the left side.  Click on the "Pending" tab you will see a green "Update" button next to your guest names on the right.  Click that button next to the guest(s) that you would like to update and enter their response right there.  Your RSVP numbers will update automatically as soon as you click "Done".