It looks like editing that rule isn't supported on the new version of the XFINITY Home website. I was able to do it using the original version of the site. Here are the steps:
1) Log in to http://home.xfinity.com
2) On the Overview tab, click on "Return to the Previous Version" in the middle of the page
3) On the Previous Version of the site, pull down the Rules menu at the top, and then click on "Rules"
4) You should see Rules for "Security System Armed" and "Security System Disarmed" - you can turn these on and off or click the trash can icon to delete them.